The Bluesound Node 2i has optical and digital coax output options. I don't think it outputs USB.
I think the digital coax (SPDIF) is considered to be superior so use that for your primary or superior source. If you have any high resolution files then they will only work over SPDIF so that could make the decision for you. I would guess your SACD player outputs redbook via digital outputs and SACD only via analog.