I'd book the later flight, and SDC onto the tight connection if you land at IAH early enough to make it. If you're in IAH for four hours anyway, it's worth a trip to the departure gate of CO2204 to see if you can make it on, provided you get in on time or ahead of schedule.

At any rate, a connection between D/E and B becomes dicey at about 40 minutes, 35 if you're starting from C. 30 is doable if you really hustle and catch a TerminaLink train right away, but I definitely wouldn't bank on it. 30 minutes mainline-to-mainline is fine, though.