I have been auditioning my E17 over the last few days and was attempting some side by side comparison of the USB input at 96/24 vs. the optical input at 192/24. I was using 192/24 source tracks purchased from HDtracks. At least for my desktop, an HP Pavilion Elite (fairly old, stock, big-box store model), the convenience of the USB definitely outweighs any potential, miniscule difference in sound quality. My optical connect is in the back of the computer and it's a bit awkward to go back there and plug in the cable every time I want to use headphones; I travel with my E17, so it wouldn't stay docked or at the desk.
Anyway, I noticed on some tracks with the 192/24 that I was getting a very noticeable hiss that wasn't apparent when I switched to 96/24. Wasn't sure if anyone else experienced that or was familiar with what it might be? The E17 is not near a monitor or cell phone, and like I said, it is only there when I play the file at 192/24.
All that said, it's pretty sweet to be able to support either of those resolutions and it really adds to the head-fi experience, especially with HDTracks Binaural+ tracks (check out one of their samplers for the low down).