short answer, you have both units, so you should try all the possible combos and listen, decide which you like best -- also, iirc the node 2i does not support usb 2.0 digital out to an outboard dac, only spdif out
my guess, having had a node 2i (but not your mac unit) is that the best sounding way will be rca coax digital out from node into the digital input of the mac
the onboard dac in the node 2i is unremarkable, to put it nicely... that was the major improvement they made when they released the node 130 (that and giving users a usb out to drive another dac)