Umm, I'm a DL elite who flies LAX-JFK often (not primarily), and upgrades are NON-EXISTANT for GMs (50K) and lower. If you're PM, you might get lucky. Even DMs (125K) sit in the back. I've missed upgrades during the most inconvenient days/times and by a lot. Also, Skypesos are USELESS.

I vote AA. Lots of good service on 767-200 wide bodies rather than 757s with DL and UA. Decent amount of service. Once you hit GLD (5 RTs), you'll be eligible to use upgrade stickers to get into J (saving your miles). I've seen empty seats in J on AA flights I tracked, and elite competition isn't as difficult as DL PLUS, AA's low tier elite status is still the most decent of the 4 legacies. AND, their miles are worth something. Partner this with 2 Citi cards and you'll be good to go.

Also, their award availability for LAX-JFK is amazing (in all 3 classes) so if these are personal trips and you're looking to defray costs with miles, that'll help a lot.