Seems to me the primary is related to the route you filed. If you're
flying direct, likely the GPS is primary. If you filed airways,
technically the VOR is primary because the airway is defined by
radials. Even though my 2 VOR's usually agree within 1 degree, The GPS
doesn't give me exactly the same course. Even so, I tend to use the
GPS as primary and use the VORs to check cross bearings. Part of the
reason is that I can enter the flight plan into the GPS and let the
STEC do the work while I monitor traffic and communications.