Hi,
I have some Z-wave sensors and need a nee hub for them. Looking for something very cost effective and can currently get a 2nd had v2 ST hub for 1/3rd of the price of the v3 / Aeotech. I have plenty of ethernet ports to plug it into so thats no concern.
Do the v2 and v3 use the same app now? Anything else I should be aware of? I’m generally a fan of buying the latest version of tech but in this case it’s hard to justify. Also looks like the v2 has a backup battery too!
Adam
The V2/2015 hub and the V3/2018/Aeotec hub both run the same firmware and have been getting updates in lock step so far.
Surprisingly, the V3 has half the amount of RAM and half the CPU power of the V2.
Some folks who need a large number of custom drivers are running into issues with the reduced RAM in the V3 hub.
Back when I installed my V2 hub the advice here was to remove the batteries. I think because they tended to be unused for extended periods of time and could leak. For myself, the hub and everything else in the wiring closet where it’s located are on a line-conditioning UPS so backup power is handled.
I wouldn’t hesitate to buy a V2 hub. Be sure it’s from the same region as you since Z-wave frequencies are different in different parts of the world.
The v2 doesn’t support Thread so if the OP plans to use Matter over Thread devices in the future, they will need another device to serve as a Matter controller and Thread Border Router.
Yes, as far as needing another device to serve as a Thread border router, but the V2, once updated to the most current software, is a matter controller. So, for example, if the V2 is all you have, you can still use matter over Wi-Fi devices. 
FAQ: What do I need to add a Matter device to the SmartThings app? Do I need a bridge router device?
Yeah, I worded that poorly. I meant if you needed a controller for Matter over Thread devices. As you point out, v2 can be used for Matter over Wi-Fi.
My response illustrates that I have decided Matter is irrelevant for now. 
Seriously, at this point it seems to me that Matter has only made things more complex, the exact opposite of how it was hyped. Maybe by this time next year.