I intend to fly both airlines equally based upon my destinations when VX starts SEA. VX for more inexpensive JFK fares and B6 for the nonstop. All things being equal I think I would fly B6 for these reasons:
Nonstop to NYC, B6 offers it, VX doesn't. If VX flew to EWR, SWF, or HPN then I would definetly fly them instead of B6 because my destination is in Rockland County.
Greater pitch, I haven't flown VX yet but my understanding is that 32<34 or 36 and that the extra pitch is good. Just common sense.
Tera Blue Cips, I love them and can't seem to get them here in SEA. The free snacks are also adequate for the redeye to JFK. I want a small midnight snack before tuning into TV.
VX UM policy, not a problem for me per se but for my family I do not like airlines that restrict UM's to up to 14 years old and only on non-stop flights. Seems like age discrimination to me. UA allows not being a UM at 12--including connections--B6 at 13, AS at 13, most airlines end at 15. If I know a child is capable of connecting alone why shouldn't he/she be able to? It is the parents decision.
Long connection times, VX's schedule has long connections in SFO--4 hours--and LAX--2 hours. VX forces 600 miles more travel through SFO, 1,000 more through LAX
TV, while I only need one TV channel, B6 offers more variety than VX

The only way VX wins is if I can convince myself to pay $900 more than I need to to fly SEA-(LAX/SFO)-JFK

My final call will be when I fly VX on the first flight out of SEA. Just two months away!