Whatever one you have more miles with currently.

If it's a 757 with the PTVs, then DL is a no-brainer.

Otherwise, go with the airline you have your miles in. CO cheap fares only get you 50% MQM on DL, but 100% on CO if booked through continental.com. I'd say CO is slightly better in coach, but not enough to swing me over 50% MQMs.

If comfort matters more than miles, why not choose AirTran (an option many people forget). AFAIK they're the only airline flying "real" jets into GPT...connecting at ATL. Business class upgrades are available to all for $40, as well.

Terminal E is CO's new flagship terminal at IAH. It serves all manner of mainline routes, both domestic and international. There is easy access to Terminal E from Terminals C & B via airside monorail. Think EWR Terminal C, but more modern, much nicer, and with friendlier people and staff.

so if i stop at IAH while flying CO, i will go through E terminal. i will have to try out that lounge :)

What a perfect analogy. That's how I would put it to compare these two carriers. ^