Yes, V2.
I can’t say about any future compatibility issues one way or the other, but it actually has more onboard memory than the V3. I wouldn’t change until you know you have to.
Matter (formerly called Project CHIP) is a new inter-operability standard for the home automation industry that they’ve managed to get all of the big players on board for. That includes Amazon, Apple, Google, SmartThings, Philips Hue, Aqara, IKEA, and a bunch more. It’s a really big deal.
The idea is that devices with the matter logo will be really easy to add to any app with the matter logo and work with any voice assistant with the matter logo. So everybody’s really excited about it, with the hope that it reduces a lot of the consumer confusion about what to buy when they’re shopping. Especially for Wi-Fi devices, which right now are all over the place as far as what they’ll work with. 
Consumer Reports has a really good FAQ article on matter
Since you said you wanted a device it would be able to work with other platforms, and matter is all about working with multiple platforms, that’s why it comes into the conversation.
But if you get a Z wave device it doesn’t work with matter, so that’s no longer relevant. Any Z wave thermostat should work with any certified Z wave hub, so you don’t have to worry about that. Any Z wave thermostat you get now should work with hubitat or Homeseer or Vera/Ezlo or any other certified zwave hub if you decide to switch in the future.
If you get a Wi-Fi thermostat, you should definitely get one that is promising future support for matter, even just within a smartthings context.
And here’s the discussion thread in this forum about matter. Again, if you only have zwave devices it’s not really going to make much difference to you.
Matter - smart home connectivity standard (formerly Project CHIP)