This is a good question, and one that hasn't been discussed to death.
DS:
-Paved the way for Wii
-Is getting huge third party games like Guitar Hero, GTA and DQ9
-Biggest system ever in Nintendo's home region of Japan
-Becoming the biggest system ever in Nintendo's historically weak region of Europe
-Breakthrough, historic games:
---Brain Training
---Nintendogs
---NSMB
Wii:
-Taking off faster than DS
-Is biggest in Nintendo's biggest region of America
-Is capable of doing much higher revenue than DS
-Breakthrough, historic games:
---Wii Sports
---Wii Fit
So I'm a little surprised, but I'm actually going to say DS.
However, Wii is no doubt Nintendo's focus for major software right now.
"[Our former customers] are unable to find software which they WANT to play."
"The way to solve this problem lies in how to communicate what kind of games [they CAN play]."
Satoru Iwata, Nintendo President. Only slightly paraphrased.
