Let's see.. you're moving from New York to LA, you'll be flying back and forth, you want elite status quick.. and you want to know which airline to choose.

I can't really answer your question without knowing a little bit more, but I'll give it a quick stab.

You get plenty of transcon service from United (Newark and JFK --> LA).

You get plenty of transcon service from Continental (Neward --> LA).

For that matter, you can fly JFK --> LGB on JetBlue but since you want elite status you're probably not going to be looking at JetBlue.

American and Delta also offer transcon service.

If you want elite status quickly, your best bets are probably American (because they offer an elite challenge that lets you fly alot of miles in a short time to earn your status) and Continental (which is offering elite status).

How much are you going to be flying? If you want to upgrade your transcons on Continental, and you fly at peak times, you're likely going to have to be a platinum (=> 75k miles/yr). That's because EWR is a CO hub.

That said, their domestic upgrade policy is nice (free). United and American only give limited free domestic upgrades (you can pay for the rest). Delta has nice upgrade policies as long as you aren't paying the lowest fares (which are ineligible for upgrade no matter what).

Personally, I like the service aboard UA's transcons the best (in C or F). That's just me.

As for the accumulation of miles, remember that all American miles count towards lifetime elite status (so you get lifetime gold with 1 million miles of any kind, including credit card miles, transfers in from other programs, etc.). United, Delta, and Continental do not offer that.

Another thing to consider is that if you pick United, you will likely also be able to earn miles and use elite status on USAirways (lots of flights out of LGA, including the DC/Boston shuttle).

Similarly, if you pick CO you can fly NW and eventually DL as well (similar if you pick DL you may be able to fly NW and CO). DL has a large route network up and down the East Coast (just as US does).

If you travel internationally at all, you might consider the airline alliances. United belongs to Star Alliance, so you can redeem your points to almost anywhere, and get almost anywhere while earning points.

American's alliance w/ BA and QF is nice, too.

No question that SkyTeam (DL/Air France/etc) will be bolstered by CO, NW, and KLM.

It all comes down to..
* what fares do you pay?
* what other destinations?
* how important are upgrades?
* what award destinations are you looking for?
and many other factors that make up an individual's choice of program...