Nobody really knows when the AA and US programs will be combined although I have speculated that 1/1/2015 would be a good time to do it. Until the programs are combined, there will probably be some at least minor differences (still some major differences now like not having reciprocal upgrades).
Since you fly a lot of US flights, I'd probably open a US account. That way you'd get the benefits of whatever status you earned on US until the programs are combined. I'd also take a look at US' trial preferred program where you pay a fee, get status immediately then have 90 days to fly a given number of miles/segments on US/USX to secure the status. Unless your company has a corporate discount program with US and could get you a status match, that would be the quickest way to get status while earning it.
At some point I'd recommend opening an AA account so that when the two programs are combined into a single AA account you'd already have the AA account.
Jim