I'd lean toward United for the following reasons (I'm assuming you're flying coach):

-8 trips a year on United would get you 1K status, which would net you 6 Systemwide Upgrade certificates, good on W+ fares (so not the absolute cheapest fares, but still very reasonably priced). You can also use miles to upgrade (with an associated co-pay). This would still work if you only make Premier Executive (4 trips a year).
-For those times you'd be in Economy, United has Economy Plus, which is an extra 5-6 inches of legroom.
-United miles tend to be easier to use. How true this remains with the Continental merger (awards on CO metal tend to be difficult to be had) remains to be seen. Given that you live in Houston (a soon-to-be hub for United), that helps out as well. It should give you some more options too on a hub-to-hub route for domestic upgrades to position up to IAD.
-United (and Continental) both use Chase - United is a Visa; CO a Mastercard; both tend to be taken everywhere, while Delta is AmEx (which I generally don't have a problem with, but some people find it harder to use an AmEx).

All told, at the end of the day, the advantages at United make this a pretty easy choice.