I think the problem with the Wii stems from the fact that once you cut out all the shovelware and halfassed third party ports, the system was just like the Gamecube. When you look at the best of its library, the vast majority of it is Nintendo first party titles such as Zelda, Mario, SSB, and the Wii Sports titles. Basically, if you liked the Gamecube library, odds are that you liked the Wii library as once you get down to quality titles, the libraries really aren't substantially different.
Personally, I think the DS and the Nintendo handheld line in general tends to outshine the available home console library. I don't think it's necessarily fair to compare the two, as handheld development times and costs are much lower than on a console meaning that more risk can be taken with less potential for loss, but in this case, the DS is the clear winner. It had pretty good third party support from DQ, to FF, to Castlevania, to Phoenix Wright, to Professor Layton, etc., as well as the standard first party support that we're used to with the Marios and Zelda (even if Zelda was a little lacking in quality). The touchscreen was great for games like Elite Beat Agents and Trauma Center, although, coming back to Zelda, it was also the gimmick that made some games far worse than they would have been otherwise.
In the end though, the DS has quality first party and third party titles, whereas the Wii largely just boasts quality first party titles. My preference is definitely the DS.
