There is a perimeter rule at LGA (According to the wiki page on LGA, it's 1500 miles, except for flights on Saturdays or flights to Denver), so I don't think any carrier can fly LGA-LAX nonstop. I agree that if you're making this trip with such frequency, nonstop will definitely be preferred.

Although I am biased toward AA, I must recommend them especially if you can make EXP. You don't say what class of service you fly this route in, but AA will soon be the only carrier to offer three class service on JFK-LAX, and once their new A321s start coming in, they should have a very decent product, including horizontal lie flat seats in F and J. As an EXP, you should have a chance of a free upgrade clearing, depending on how far ahead you book.

As of right now, AA's SWUs are also the most flexible, in that they can be used on any fare. UA's SWU can only be used on W fares or above, which can get quite expensive. DL's SWUs are also considerably more restrictive than AA's.

Yarhead makes a good point that it will depend on your own personal preferences, and what is important to us may not be as important to you.