I have done BWI-IAH a few times this year. It is a good route to do as a Plat. Usually on the morning flights, there are still several seats for Golds and Silvers. I have been EUA'd everytime but once. Avoid flying this on Friday afternoons. I got my upgrade on that Fri afternoon flight upon check in at the airport, no EUA in the box before that. There were only two seats left in First, for me and another Platinum. Your best chance is on the morning flight.
If you cant do that, then go for the 738 or you may want to try a 757 with 24 seats in First.

If your EUA doesnt come through even after check-in, be pateint. As of recently, CO seems to be holding seats back on this route.

WASJETBOY