You didn't mention where you are flying to for work. That to me would be the biggest deciding factor. I may be biased being a long time UA flyer, but out of SFO, for schedules, I would go with UA - and for work purposes, that would be my biggest determining factor.

That said, you also say you will be flying 1-3x/month. If you average 2 (I'm assuming RTs) and have the odd personal flight, you should get very close to qualifying for AA Plat or UA 1P on segments. Make a few connecting flights and you're there.

Being 1P or lower in SFO won't get you very many upgrades, esp to IAD however.