They are both disappointing so its more of a question of which disappoints the least. Lets see.....
Brawl:
It was toned down for newbies and casual players. Button mashing will actually get you somewhere as suppose to in Melee where every punch and kick is vital and skill was commonplace. In Brawl, Veterans are crippled to the point where they are no longer playable and don't stand a chance against the newcomers, like Diddy K, Sonic, Lucas etc. A example would be Marth's charge B attack, which was replaced with a stab with no reach and sorry damage instead of the regular overhead charge attack. Another example is Links spinning attack, which was essential for Link to stand a chance because it made up for his lack of speed, was also neutered and replaced with a pathetic charge attack with slow charge time and minimal damage. I could go on forever, seriously. Don't get me started on the items, final smashes or terrible story mode.
M-Kart
Wii:
Also just as bad with casual game play as a major focus. The "rubber banding" is ridiculous, especially in the harder difficulties. Winning in a game should never depend on
pure luck. New items are laughably bad.
That said I would say M-Kart Wii is a better experience in the long term. Sorry for the hate filled post, I was waiting to get this off my chest for a long time. I've been a hardcore smash/kart fan since the beginning and it es me off that Nintendo is mixing hardcore with casual.
Kotaku- "50% cake and 50% crap is still 100% crap"