In terms of platform compatibility with the Romi, any of the 40-pin GPIO models should work (but we’ve only tested the 3B and 4).
Integrated WiFi is a very nice feature to have for the Romi; the web interface for WiFi settings we know works with the 3B and 4 but we’ve not tested it with a USB dongle.
One downside of the 4 is it really likes to have a heatsink, particularly if you’re doing image processing, and there’s minimal clearance on the Romi to fit one. The 3B is also nice because PhotonVision supports GPU acceleration on it if that’s something you want to explore (so PhotonVision overall performance is actually better on a 3B than a 4). If you’re home-rolling vision processing (which probably means most operations on the CPU), you might want to go for the 4 instead, as it definitely has a beefier CPU.
You aren’t going to need much RAM for Romi or vision coprocessor use. 1-2 GB is fine. If you want to future proof it for other use cases (e.g. a desktop replacement) you might want more.
My recommendation today: get a 3B+, or if you’re future proofing for your own vision processing, get a 4 with 2 GB.