I have experience of both (just fitted Dension Gateway 100 into my A3 yesterday). There's very little in it. The advantage that the Dension has is that albums, artists, songs etc can be selected from the steering wheel and that text is displayed in the DIS. The xcarlink can still change tracks, but has limited control compared to the Dension.
Personally, I like to have my iphone mounted where I can see it anyway, and like to control the music on the phone itself. This means that, for me, the extra control and text from the Dension is not that much of a bonus. If you prefer to hide your ipod then go for the Dension.
It's difficult to say which has the better sound quality; I had the xcarlink fitted in my Honda Accord which didn't have the best stereo to start with, especially compared to the Bose in the A3.
As for fitting, it is exactly the same for both units. Pull out stereo, disconnect quadlock connect, slip in the new cable loom and Bob's your uncle. Plenty of room behind climate control panel for the new kit.
