HI rscosworth
I've just gone from a BMW to a GTi. I had professional media with professional speaker system, and I've gone to the standard VW Discovery Nav with Dynaudio sound system.
I would say, and that's probably because I'm not yet fully up with it (only had it 4 days), that the BMW satnav was marginally easier to use, especially on the move, but the Discover Nav is more feature rich and has better directions such as prompting you to two manoeuvres ahead. Her pronunciation is a bit laughable at times though.
USB works fine as does SD, though you might need to change the layout of your files if you have playlists. The quality of the sound system is pretty much on a par.
The GTi has plenty of poke, and I've just come from a Z4 sDrive28i. It's not quite as fast as the Z4, but much, much nicer to drive, and when you don't want it to be a hot hatch it's really easy going. Just been to Wales and back, mix of motorways, town centres and A/B roads, and got 35.3mpg average (Z4 used to come in around 29 on the same trip).
For what my advice is worth, I'd go for it...