UA is out of the question. Their service out of NYC leaves a lot to be desired. That may change if UA/CO becomes a reality, but then you would have to deal with EWR. That leaves DL or AA. AA is better for international upgrades, but at the EXP level (100K+). AA does not offer unlimited complimentary UGs domestically. Out of JFK, DL has the best program for low or mid-tier elites, IMO (but as you know, I'm biased ) I'm at around 80% for eligible flights, and have received a couple TATL op-ups as Gold. UGs are relatively easy to come by on JFK>non-hub routes. Book wisely (avoid heavy travel times and days), and DL provides an extremely valuable program to NYC-based travelers (excluding TATL/TPAC routes). That said, UGs will be rare if you book the heavy holiday weekends once you attain FO.

TATL: You CAN upgrade to BE on TATL flights, but the required M fares are very expensive. Many on this board have predicted DL will never change this policy, but I can see DL adjusting their UG requirements down the road to allow UGs on lesser fares (H? Q?) with an increased mileage/SWU requirement, and of course with a limited availability.