I'd be surprised if there was much of a diffrence. The spdif is buffered and reclocked. I will try the USB against my offramp 5 converter myself at some point and predict the offramp will be on Audiogon shortly. However, I bought a unit with USB, and the offramp / battery / cabling is $5k msrp, so for me it would make sense to go the USB route financially.
I am looking forward to read your considerations.