There are a lot differences, some are good, some are bad, depends what you want.
The EU unit does not work with XM/Sirius and it does not support AM very well, FM is fine though. The EU unit needs a patched US map which requires the modification of large files. The EU unit does not power off with ignition, it powers off when removing the ignition key. The EU unit is known to work with OEM TV tuners and confirmed to work with 3rd party video/audio in adapters. Video in motion is only possible by cutting the speed wire (4B0 unit) or adding another 3rd party module (8E0 and 8P0 units), both prevent navigation from working properly. There are guys in Germany who sell a VAG-COM mod for video in motion but you have to have the OEM TV tuner and you have to bring your car to them to make the change. This method is supposed to support both TV/Video in motion and Navigation at the same time.
The US unit works with XM/Sirius but requires new SAT receivers (you cannot control your existing SAT receiver through the US RNS-E although audio is working fine). The US unit has not been confirmed to support any kind of audio/video input.
I'm sure there are ore but this is what comes into mind first...