Wii and DS provide better gaming experiences.
LOL we can argue over more accessible. I think that would be a valid opinion. This also results in higher sales which is fact.
But this doesn't result in better gaming experience. Since games are what is important I would argue that the motion controls result in worse games and therefore worse gaming experience. It may be more mainstream but thats a different matter. I would call it dumbed down.
I mean have a look at the games. The good Wii games: Zelda, Galaxy, Smash, Mario Kart all have at best very small dependence on Motion controls. The Gamecube version of Zelda is just as good. Collecting starbits in Galaxy is at best annoying, and Smash and Kart can be controlled just as well without using motion controls at all.
So the only great game that really has a significant upgrade through motion controls is Metroid.
Whats left is the impressive and fun Wii Tennis and lots of crap: Wii Play, Mario Party, Carnival Games, Lawn Games, Rabbits. They may be fun for a while and as a party game but this is the equivalent of American Idol music. Its accessible, many people buy it but if you know a bit about music you do not hear it.
The power of PS3/360 brings huge cities to life in Assassin's or GTA. You can play a live movie in Uncharted or get the perfect atmosphere of Call of Duty. This may not be able to break the casual gaming juggernaut that is Nintendo. But it simply is better gaming. (For a given definition of gaming)