For LAX-NYC, UA has the best product. So if you wanted to use miles or stickers for upgrades, this would be a good value, but you would only get upgraded part of the time.

AA and CO are also good options for LAX-NYC.

For international UA's international product is equal to or better than AA's - CO it better, but not good for upgrades (requires high fare and more miles).

AA allows upgrade for almost all fare classes international but charges a co-pay; UA restricts you to buy a higher fare class so you can upgrade, but no co-pay.

AA is slightly easier to upgrade LAX-NYC than UA. AA also charges less than UA for 500 miler upgrade "stickers" if you wanted to buy more to upgrade more so you could request an upgrade on all of your NYC trips.

I find Delta, Northwest, and US to have too many issues...not as quality as CO, UA, and AA. CO is harder to upgrade so that leaves you with AA and UA. UA has a slightly better product on your routes but AA is slightly easier to upgrade.