I had a 3+ black and now have a 4 silver.
The 3+ black would probably be the best one to search for used because it came with the remote control. IMO having the remote is super nice for use on the boat. the gopro app is okay, but uses more battery and isn't as reliable at maintaining a connection. Plus the remote is VERY easy to hand to about anyone to have them control the vid.
I went the the 4 silver because I'm a sucker for technology and wanted the LCD touchscreen.
My buddy just went the "best value for cheap" route and got the gopro 3, then bought a remote after the fact. Personally, I like how the gopro 3+ black and newer have the superview because it gives a widescreen without cropping any of the image:
But if that's not important to you, then you can get a little more value with the 3. Keep in mind that the 3 is bigger, so the newer cases won't work. I think they all have different batteries too. But get extra batteries/charger/mounts on ebay anyway for cheaper.
What's your budget? It'll take about $300 typically to get a used 3+ black (includes remote).