In my area Regular and Super cab's are rather rare - almost all the used and new inventory are "crew" offerings.
Presently I have a super cab model. Rarely (if ever) is there room for anyone to actually sit in the rear seat as I have the seat (and the floor) filled with larger truck and camping items that I'd rather not find room for in the camper.
While a 'crew' cab might offer even more inside storage space, personally I have a 'mental block' in considering one over a super cab as I already have enough storage space (the DW says we bring ENOUGH stuff with us) and we really have no need for more cab space to fit more people.
We already have to park our rig in the "north 40" because of turning radius so I cringe on the thought of getting something WORSE.
YMMV: with all the stuff we have packed in the rear of the super cab, I'd never consider getting a regular cab.