For a listing of which CO fare classes only earn 50% see:

http://www.nwa.com/freqfly/earn/airfl.shtml#continental

Your "I" outbound is 50%, but a "U" return is 100%. Also note that the 50% is only for elite qualifying miles (i.e., the annual odometer that determines your elite status); all CO coach fares still earn 100% banked miles (i.e., the miles you accumulate like currency that you can use for award tickets). If you don't think you'll make elite in 2006, then there is effectively no difference between flying NW or CO.

Also, IMO CO's IFE and "sandwich" are pretty bad. Their IFE is usually a terrible movie followed by a bunch of advertising dressed up as television programming. Sometimes you'll get lucky and get to watch sitcoms, but usually no. And the "sandwich" lately has been mystery meat of the worst sort. I'd rather bring my own sandwich and book or DVD to entertain myself.

As for the rental car question, all IAH rental cars are located at a centralized facility far away from the terminals. You'll have to take the shuttle bus to get there no matter at which terminal you arrive.