This is a really complicated question right at the moment and it may change a whole lot over the next few months because Apple just gave permission in June 2017 for a software – only HomeKit integration.
Up until now any devices which wanted to be HomeKit – compatible had to have a special encryption chip included which was expensive and meant you couldn’t retrofit existing Devices. But by allowing the software only option, it is possible that by December 2017 or at least March 20018 we will see a whole lot more available HomeKit – compatible devices. The Z wave alliance has already said that there is a HomeKit/Z wave hub going through the certification process, but they won’t say which brand it is.
All very exciting, but also a bit maddening, because you can’t know exactly what’s going to be available in the first wave. 
Apples and Porcupines
Every system has pluses and minuses, and that includes both smartthings and HomeKit.
HomeKit
The HomeKit advantages are that it’s very reliable, it will work even if your Internet is down, and the devices are high-quality. And it’s very simple to set up your system.
The biggest disadvantage is that right now they’re just aren’t very many devices available although as mentioned in the first paragraph, we are expecting kind of a flood of new ones over the next six months.
A potential HomeKit disadvantage is that the rules that you can set up our essentially limited to “if a or B then C.” This is a structure that works for a lot of rules for a lot of people but definitely doesn’t cover everything. If you start thinking about rules like “if A and B or just C but not D unless it’s every alternate Thursday between 10 and three,” what we call a “stacked conditional” there’s just no way to do that HomeKit. HomeKit can do “if there’s motion in the living room and I am home, then turn on the light.” But it can’t get much more complicated than that.
SmartThings*
The SmartThings advantages are that it’s extremely flexible and very easy for customers to add custom code and extend the system to devices which Samsung doesn’t even know exist. And you can set up stacked conditionals to your heart’s content, a dozen levels deep if you want. So you can use more devices, more kinds of devices, and much more complex rule sets.
The disadvantages of SmartThings are that it’s not always easy to figure out how to access all that power. ( although people in the forums are always happy to help) and they just haven’t solved the reliability/stability issues yet. It is a company priority that they do so, but the fact is every few weeks, something that used to work just fine breaks. And they may fix it right away or they may not, depending on the reasons why it broke.
Also, it’s a cloud-based system. They can also push out updates at any time and there’s no way for us as individual customers to postpone or refuse those updates. They just happen.
So HomeKit and smartthings are very different platforms. HomeKit is all about security, stability, quality. Which means they may limit the available features and they definitely limit the available devices. SmartThings is all about versatility and flexibility, which means you can buy nine dollar sensors from China and create usually complex rule sets – – but there’s no guarantee that what works on Monday will still work on Tuesday. Most of the time it will, for most customers, but they’ve had at least one major system issue every month in the last 14 except i think for January 2017. A lot of times the company fixes the issue in a few hours or at least a few days. Many times there’s a workaround that somebody in the forums comes up with, but it’s a very different kind of experience than relying on HomeKit.
Which is why you often hear me say the different things work for different people. They do. I myself am quadriparetic, use a power wheelchair and have limited hand control. If the lights don’t come on and the fix is just to open the sensor case and pop the batteries and put them back in, I can’t do that. I have to pay someone else to do it. So for me, reliability is a huge issue. Other people may handle glitches like that without even thinking about it.
Sorry that answer was so long but I’m just trying to make clear way different people will report different experiences.
Amazon Alexa
I seriously love Alexa. As someone who is quadriparetic, I’ve tried most of the voice technology out there, including google home, and in my experience nothing comes close to the far field recognition technology in echo. I use it all day every day and it has seriously changed my life.
As I mentioned before, I also use Siri, but if there something that they both can do, I will always use Alexa first.
Using Both SmartThings and HomeKit
You can absolutely use both smartthings and HomeKit, we do at my house. Over the last year we have moved more and more critical use cases over to HomeKit because I just need to reliability. But there are some things that SmartThings can do that most of the competitors in the low-cost price range just can’t. For example, we have it set up so that we get a notification if the guestroom window was left open and rain is expected. Very useful, and there’s no way to do that in HomeKit. And if SmartThings skips a beat on that particular day, it’s not a huge issue, when we hear rain start, usually somebody says “hey, is the guestroom window open?” And we check it then. But it is nice to have the early notification.
If you want to use both you have to choose devices which are integrated with both, or you have to choose which Devices will be on which system.
Easiest way is to start from the Homekit list, just because it’s so much shorter. 
Lutron Caseta switches
Serena shades
Ecobee Thermostat
Philips Hue Bridge and its bulbs
Nanoleaf Aurora Light Panels
Logitech Pop Button
IHome smart plugs
All work with both. They also all have their own integration with Amazon Alexa, which is nice.
But there are no cameras, smoke detectors, door locks, or sensors which work with both HomeKit and SmartThings at the present time. (Again, that may change over the next few months because Apple has changed the way HomeKit integration can be added to devices.) so for now, you have to make decisions in that area.
There are some companies like fibaro that make a HomeKit model of their sensor and a zwave model of their sensor but even though they look identical, they can’t talk to both systems. You have to choose one or the other for each sensor. But you might have some of the homekit model and some of the zwave model if you were running both platforms, it just depends on what route you want to set up.
Where to Start?
So where do you start? I think the first question has to be how important is reliability to you. Because, honestly, that’s the main weakness on the SmartThings side. If you’re OK with a system that probably glitches two or three times a month, Smartthings may be all you need. If, like me, you want an MFOP (maintenance free operating period) of at least six months for your critical home automation use cases, then you’re going to need to add in at least a “Plan B” platform.
If you’d like to see what some of the typical glitches can be, there’s an official status page, but they tend to only put the really big outages on that.
https://status.smartthings.com
You could also look at the “first bug reports” page in the community – created wiki, which will include all the little problems that maybe only affect one particular device or only some customers. It’s not that everything listed there will affect every customer all the time, but somebody got hit with it or it wouldn’t be on the list.
http://thingsthataresmart.wiki/index.php?title=Bug:_First_Reports
The issues with SmartThings aren’t subtle. I usually tell people who are interested in it to go ahead and buy the hub and a couple of devices from a place with a good returns policy and do one room, preferably one you use a lot like the kitchen or the living room. Then just use it for three or four weeks. If it doesn’t meet your own personal requirements for stability, you can return it. But be honest with yourself about how much maintenance it requires every week.
At the end of the trial period, you can decide whether it’s a platform you want to move ahead with, a platform you want to use in addition to another system, or whether you just want to move on to something else.
Again, different things work for different people, so there’s no one right answer here.
The only thing you don’t want to do is rush out and buy $800 Worth of equipment and then not install anything for two or three weeks, because then if it turns out to be something that doesn’t work for your household, you may miss the return period.
Just my own opinion…
. I am able to do everything through Alexa; routines, automations, etc. I didn’t even bother with HomeKit because using SmartThings really gives me everything I need.