It really depends on each specific use case.
Zwave plus is a big technological jump for Z wave from the previous generation.
The most immediately obvious difference will be longer range. We normally use a rule of thumb of about 40 feet in a typical U.S. House. With zwave plus, that goes up to 60 feet. Since zwave (all generations) is limited to four “hops” Per message, having that extra distance can help a lot in a really big house or if you’re trying to get messages in from a shed or detached garage.
In fact, if you’re planning out your whole network, you may find that your total costs are less if you select zwave plus for the backbone repeaters because you don’t have to place them as close together.
Part of the Z wave standard is that devices be backwards compatible, so you can use a zwave plus repeater with a Z wave classic battery operated sensor, or vice a versa. 
So the good news is you only need to buy zwave plus devices where you really need them. You can fill-in with less expensive zwave devices if you want to.
There is also a hidden cost savings because zwave plus devices have better battery life. Probably at least a third better for battery-operated sensors, and maybe as much as 50%. So you do get some cost return that way, especially for “busy” devices which have a lot of events.
As far as specific technical advantages that you might not see, the most important is probably a change in the pairing method which will allows you to Pair more devices in place. this is nice for light switches, in particular, which are more than one hop from the hub because then you can wire them into their ultimate location and as long as there are zwave plus devices along the path to the hub, you can probably pair in place. Otherwise you either have to wire it to a power source near the hub and then move it to its true location after pairing, or put the Hub on a really long ethernet cord and carry it to the switch.
There is also a bandwidth and channel management improvement, but i’m not sure how much practical difference that’s going to make in most installations. It may help somewhat in apartment house deployments where there are a lot of systems that are physically close together.
Zwave plus should also allow for “over the air” firmware updates, again nice for light switches or anything else wired to mains power, but I don’t know if smartthings is supporting that yet.
So is longer-range, better battery life, and easier in-place pairing worth the extra cost? That’s an individual decision, not just for each house, but really for each device. 
Some terminology
And a note on terminology: the most current generation is zwave plus. You will also see it referred to as 5th generation or series 500. It’s all the same thing. The series 500 chip is the fifth generation of Z wave and its marketing name is “Z wave plus.”
“Classic zwave” or “zwave Classic” now usually means the third and fourth generations, or the 300 series and 400 series chips.
“Early zwave” or “1st gen” usually means the series 100 and series 200 chips. You rarely see those devices anymore, but you might see one from an original installation. I wouldn’t recommend buying these, no matter how cheap they are, as there were a lot of improvements that came in with the 300 series.
I would still buy fourth-generation, and maybe even third generation, devices if the price were right and it fit what I needed for that particular use case. For example, The gocontrol security essentials package, which is zwave classic, is on sale at a lot of places right now, probably because they’re getting ready to bring out a Z wave plus version. This is two contact sensors and a motion sensor. Sale prices range from around $20-$35, all good prices. If I needed some more Z wave sensors, I’d definitely be looking at those right now because it’s obviously a great value. 
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