Sorry, I don't have access to a Sync 3. From a quick look at the tutorials, it looks like Sync 3 refined the look of the UI, which looks nicer, but is probably harder to use, because the buttons looks like they have smaller borders, so it would seem easier to hit the wrong ones. Also, it looks like there's more buttons on the screen, which again makes it easier to hit the wrong ones.

Sync 2 looks like junk, but if it wasn't so slow, I think that would be fine.

Sync 2 had a hook for addon apps, but never got any. Also, they killed the thing to send directions to the car before I even got my car (it seemed like it was poorly thought out anyway), and the vehicle health report too. Still lets me use an app to find the car / unlock the doors / etc; but only if the car hasn't gone into deep sleep to stop it from killing the battery.