Mario Kart 7 is better IMHO. Some features in Double Dash annoyed me. I didn't like having to think about switching positions in the kart to make full use of my items, and some of the character-exclusive items choices didn't seem fair. Some of characters had exclusive items that were equal opportunity items in previous games like multiple shells and bananas peels.
I'd say 7 as well. DD(obviously) lacks retro tracks and kart customization which, for me, makes the later games and 7 stand out. Comparing the rosters, I probably prefer DD, but I give credit to 7 for adding some good new characters. So overall, I'd say 7. DD has a great co-op racer though, my cousin and I put hours into it.