I guess it'd depend on where you're coming from in town! The hard part would be the traffic. Distance does not equal time anywhere around downtown Dallas.
RBD is a good facility, crosswind runway, competition between FBOs. Easy to get to from I-35E or 67. I call them Redbird tower all the time, but they recognize my tail number, I guess.
GPM is a quiet place, haven't been to the SS pumps but will check them out. About 5 minutes of TX 360. Small but decent terminal building, free wi-fi, at least in the past. Report over the Joe Pool Lake dam (SE) or marina (S) on call-up. GPM didn't used to have radar in the tower cab - heck, they're doing good to have a radio and a phone!
They're both good airports. Learned to fly at GPM and I overfly RBD about every weekend and will stop for gas if it's cheap.
And just for grins, if you want to play turbine pilot, I think Jet Aviation at DAL (located next to the terminal parking garage) doesn't charge a ramp fee for picks up and drop offs, just off 31L...
TODR