In my opinion, Mario Kart Wii will have stronger legs than GT5. If you look at the sales of GT4 after the first 6 weeks of its release, the sales were around 90k worldwide and Mario Kart Wii's sales were around 120k by week 9 or so.
So Mario Kart Wii's weekly sales are around 100k right now.
10 weeks after GT5's release, the sales will probably be between 80-90k worldwide, so there will not be huge difference in weekly sales.
edit:
However Polyphony Digital might release DLCs for GT5 so that could increase weekly sales.
Anything could happen.
Let's not forget a possibility of Sony bundling the game which could increase the sales of the game.
TO GOD BE THE GLORY