I believe you are right, Gran Turismo sells because of the amount of polish that goes into it. And in terms of the highest sales of a single game GT takes the cake:
GT 3: A-Spec comes in at 14.87m
while the MK series has:
Mario Kart DS at 12.66m and
Mario Kart Wii at 10.46m
This could of course change as MK:DS and MK:Wii I think both have a change at the big 15 while GT5 will suffer from the relativly smaller user base of the PS3.
Now if you compare GT and MK in terms of total sales the winner shifts sides:
MK Series Total: 54.16m
GT Series Total: 49.92m
These numbers are all based off of VGChartz data of course and can be found:
Here and Here
Currently dreaming of: DKC4 or Sonic the Hedgehog 4 (classic 2D platformers) for WiiWare, Smash Bros. for DSi, New Super Mario World for DSi, a Wii remake or true sequel of Final Fantasy Tactics.
One down, hopefully more awesomeness to come.
