That's quite a difficult choice.
Both consoles have their good points and their useless gimmicks. If I went with the Wii, I'd be forsaking portability, which is a big minus for me. Also, it seems to me that for every third Wii game you have to buy a new controller accessory
On the other hand, I'd be getting a number of great exclusives as well as longer games with more complex gameplay and better visuals that are impossible on the Nintendo DS. I've also noted that Wii games, even the goofy ones to an extent, have more engaging stories than DS ones - many of which don't have a story at all.
If I went with the DS, it'd be portable, region free, and backwards compatible with the GBA - all huge plus points for me. I'd be losing out on the story-driven games on the Wii, though, and not all games have a control scheme that suits me since some of them, especially the cheaply-made ones, try to make unnecessary use of the touchscreen. Many story-focused games don't have very good stories on the DS. The visuals would obviously, and understandably, be inferior. However, the good DS games try to make up for the lack of an engaging story with their gameplay, and unique DS games make up for the inferior graphical capabilities of the consoles with their art style.
I would choose the Nintendo DS, overall, though if I knew I was missing what the Wii had to offer, I'd feel dead inside
What about you, Marc? What would you choose?