I tended to trust Firewire more than USB however, but I guess my concerns stem back from the USB 1 days, when it was barely up to the job of punting a stereo 16/44 track through dependably. Never had a single problem with Firewire. But then I expect USB 2 is rather better now too?!
I'd be giving the RME Babyface a good look also.. I've not tried one, but they do have a solid reputation for quality. And even the Babyface is a lot more flexible with regards to IO than either of the Duets.
I do need more IO now, so i've been looking at the Babyface and Fireface 400 myself. Little point in a new Duet 2 for me, and the Ensemble is a bit spendy..