Is there a reason to buy V3 and not V2

There are hardware differences other than the WiFi. The V3 has half the RAM of the V2 (256 MiB vs 512), and half the CPU speed (500 MHz vs 1 GHz), so in that respect the V3 is a downgrade rather than an upgrade. The V2 also has battery back-up, which the V3 does not.

The WiFi connection option can help with installing the hub in a more central location for some people, but otherwise I feel the V2 is a better hub.

There are no differences in what sensors can or can’t connect to the V2 vs V3 as far as I know. The V3 hub and SmartThings mesh WiFi hubs are both Zigbee 3.0 compliant, which the V2 is not, but the primary difference is that the V2 does not allow QR code on-boarding of newer Zigbee 3.0 devices. You can still connect Zigbee 3.0 devices to either hub, just not by using a QR code with the V2.