Assuming you are talking about the Z wave “Decora smart models”, that’s the newest one that communicates directly with SmartThings. There are several differences between that and the older Vizia models.
To begin with, the look is quite different. The older model is unusual in that you press on the at the bottom of the switch whether you are going to turn it on or off. It will not match your non smart switches.
The new model looks like the non-smart Decora switches. You press at the top to turn on and you press at the bottom to turn off. It will look very similar to other non-smart Leviton Decora switches that you might have.
New decora smart zwave model
Another difference you might notice in daily use is that the newer line is better at handling low loads, such as a single LED bulb. This was a known issue with the older line.
engineered to function with extremely sensitive, low-wattage light sources, such as a single LED bulb in a hallway or a strand of holiday lights, ensuring LEDs do not glow when off
There are a number of additional technical improvements which come with it being a Z wave plus switch versus Z wave classic. You might not notice those in every day use, but they do make things a bit easier if you had to remove and re-add to switch to your account later, and it will make things run a bit more efficiently. The Z wave plus devices also have a longer range than the older line.
Speaking just for myself ( with a typical engineer’s bias), I would say unless you’re in love with the unusual look and function of the older line, I would get the decora smart. But it is also possible you might save some money if you find a retailer carrying the older line, you would just have to check and see.
(note that there are multiple model lines under the “Decora smart” name which all use the same case, so they look identical. Only the Z wave one will work with SmartThings. The others include a Wi-Fi model, a HomeKit compatible model, and a Bluetooth model. But to work with SmartThings, you want to get the Z wave model.)