Sounds like you have some fun projects ahead! Lots of us use both Homekit and smartthings, it’s a fairly popular combination if you already have an Apple device to serve as a hub for that platform. 
As I’ve mentioned before, I use a combination of smartthings, Apple Home, and Alexa at our house, and it works well for us.
Alexa just because it’s still a better voice assistant than Siri, and because at the time of this writing, it works with a bunch more devices than the other two. Just some examples, our central heating system has an Alexa integration, but nothing else. Same with our robot vacuum. And our automated kitchen faucet. And the microwave.
(I am quadriparetic, use a wheelchair and have a limited hand function, so home automation isn’t just a convenience for me: it’s a necessity to get through my day.)
Back in 2015 I had a lot of reliability issues with smartthings, so I started moving my mission critical automations to HomeKit. Since that time, HomeKit has gotten better and better in terms of the number of candidate devices available and the complexity of the rules it can support.
And smartthings hasn’t gotten much better in terms of reliability. 
In particular smartthings can and does change functionality at any time without announcement or documentation. You’ll see several threads in the forum every month from people saying “this worked great for years, I haven’t changed anything, but now it stopped working.” Sometimes there’s a workaround, sometimes there isn’t, but it takes time and effort just to keep up with all the unannounced changes. 
Another important aspect of HomeKit for me is that everything runs locally except Siri, so if the Internet goes out (which happens several times a year in my area.), everything still works. And I’m not without voice control: I can still do voice navigation of the app itself if needed. When you can’t just “walk over and turn on the light switch,” this kind of reliability becomes more significant. I know there are other homes where it won’t matter as much.
And finally: I love my Apple Watch. 
That is all. LOL!
If you’d like more details about my setup, I have a long project report here, but it’s not really as relevant to your specific questions. It’s more about how I chose exactly what to automate in a way that could meet my budget.
Adding Home Automation in Phases: my limited investment strategy
——
Beyond that, definitely, matter will make things easier with regards to reliability for inexpensive sensors, and some other devices, as well as making more devices available to use with both smartthings and Apple Home. Aqara is a perfect example. So I certainly think that’s a good trajectory.
I don’t have a lot of other tips in general. I try to choose devices which work with all three of my preferred platforms. (Alexa, HomeKit, and smartthings, often with Alexa as an intermediary). But that’s not always possible, and then I select on a case by case basis, depending on the priorities for that specific use case.
I know a lot of people like the smartthings app the best: unfortunately, its voice navigation has been broken for several years, so I mostly use the Apple home app and use the alexa app occasionally, and the ST app as little as possible since I have to have somebody else do that, so I’ll leave those discussions to other people.
