I personally like the Wii however my son is 8 as well and he loves the DS. He has a DS and I have to say that it is a great device to have when on car trips or in waiting rooms etc. around town. You can find games that include chess, checkers, word games, etc. The games don't just have to be Mario Cart and so forth. After attending a chess tournament yesterday and watching the children in attendance we found that those of us that had our children's DS had the best behaved children. The others were running around like crazy and the parents were not as involved with their children obviously as we were. This is just another example of why the DS is a great option.
Wii is definitely a great game/s but there are less expensive versions out there. We have a Sport Challenge. My son loves it. We will probably break down and get the Wii eventually but for now DS and our less expensive version of Wii works for our family.
by the way my son is 8 or almost next month