I definitively vote for DFW. There's way greater activity than in IAH, I just love when I'm on final approach into this giant and I'm able to see not one, but TWO other planes flying parallel approaches into different runways (like when you're landing on 18R and there's traffic on final to 17C and 17L). Personally, I never get bored of so many AA MD-80s. Besides, there's still the big number of wide bodies operated by both AA and DL (767s, 777s, DC-10s, MD-11s). On the inside, AA's terminal is much roomier than CO's at IAH. However, I'll have to admit I like IAH's IAB better than AA's international arrivals at Terminal A.
I guess the bigger an airport is and the more traffic it has, the more I like it.
"... for there you have been and there you will long to return."