AA refers to them as "500 Mile Upgrades." eVIPSs are systemwide upgrades on AA.

AA's upgrades are prioritized by status and then time of request. EXP's require 0 stickers per 500 miles for their own upgrades.

Importantly, UA requires a W fare or above (for coach travel) or J/C/D (for business class travel) to use a systemwide upgrade internationally. This can have nontrivial cost differences (with no guarantee that you'll actually clear). Most of my UA systemwides this year have gone towards domestic flights (where they work on any fare).

For now, AA continues to offer a comparative advantage as its systemwides work on any fare.

Btw what credit cards do you have? I'm not sure how familiar you are with credit card churning, but since no one has mentioned it, you should apply for all of these airlines' respective credit cards to maximize earning potential. I'd at least 2 of them since the consensus is that you should aim for top tier on 2 of these programs (totally doable given the number of miles you'll fly in a year).

E.G.
CitiAAdvantage VISA and/or CitiAAdvantage AMEX
Chase United Mileage Plus Explorer
Delta Skymiles AMEX

You might want to consider the higher end versions of the Citi AA and United Mileage cards, the benefits might be of interest to you with the amount of flying you're going to do (for instance lounge access). These cards will help you accumulate miles even faster when you use them to purchase tickets from the respective airline (for instance 2x) which you then use on partner airlines for your personal trips. One thing though is to check your company policy and make sure you are credited the miles to your FF accounts (and retain them) since they are reimbursing you/paying for these flights. I know some policies don't allow you to keep them, but they vary from company to company.

Overall it sounds like you can't really go wrong, its just a matter of maximizing what you can get out of your work travel. Curious, what do you do that allows you to fly back and forth to LA? I love LA and would be thrilled to be able to go every weekend (well maybe not every single one).

+1 Lots of great info here by Yarhead and other FT members, but the bottom line is that I think Yar hits the nail on the head with this recommendation. Flying back and forth so regularly, the OP might well come to care a lot about upgrades, in which case AA clearly has it over UA. And you also might care about redeeming your accumulated miles, in which case AA clearly has it over DL.

Yet another factor is that UA has become horrible at dealing with the inevitable problems that stem from weather or mechanical difficulties. Putting aside the temporary labor issues with its pilots, AA is much better at handling those irregular operations. I can't say how AA compares with DL in that regard.

My bad, thanks for the corrections and clarifications.

Do you live in NYC as well? The JFK-SFO/LAX routes are reasonably price competitive if you grab tickets when the fares are good. Book first, ask questions later :D

On that note, I suspect that I'm on track to spend more weekends in either LA or SF than NYC this year... (and I live here)

Yes sir, I do, smack in manhattan. I've definitely seen those almost MR worthy fares from NYC to LAX and was tempted to buy them speculatively, but that's not the smartest thing for me to do since my work is not as flexible for that kind of travel. But pretty awesome that you get to spend so many weekends on the west coast! Is it for business or pleasure?

Pleasure. 193K BIS or so this year, only 5K of which has been for business.