This is all my opinion, I still love New Super Mario Bros Wii, feel more then welcome to disagree, but here are my points on why I find Mario Kart Wii more fun then the New Super Mario Bros Wii.
1. Mario Kart Wii has a greater variety of options - Mario Kart Wii has 124 characters to choose from (mii's included), solo or team mode, online tounaments, online races, online ghost races, a variety of engine size, CPU difficulty, karts/bikes, and the list goes on and on. New Super Mario Brothers Wii has Adventure, Free-For-All, and Coin Battle. Oh yeah, and 2 generic Toads, Luigi, and Mario. Nothing else.
2. Mario Kart Wii has more quality courses - Don't get me wrong, NSMBWii has fantastic levels that are fun and thrilling. But it doesn't have the same WOW that a majority of the MKWii courses seem to have. What levels in New Super Mario Brothers Wii had you falling down a waterfall and into a tunnel? What level had you exploring the treetops and making tremendous jumps after a giant barrel blast you off? What level had you swerving from frantic traffic as you try to hit your target with a shell? Only Mario Kart, baby.
3. Mario Kart Wii uses motion control properly - There is no better feeling then turning the wii wheel to alter your character's direction. Except maybe the feeling of pulling a big-air trick and getting a speed boost. NSMBWii's control consist of miving platforms and franticly waiving your wii remote up and down like a horney teenager until you can POP the bubble. That's all the work and none of the fun. It is not satisfying.
4. Mario Kart Wii actually teaches responsibility - Take my little brother for example: He is only 10 years old and more of a sports guy, so he stinks at gaming. In Mario Kart, if he falls, he fails and needs to work hard by grabbing item boxes and performing better to get his rear in gear. With NSMBWii, if he's about to fall, BUBBLE! He gets to continue with me, despite his lack of skill. He gets all the glory and reqard for none of the work, skill, and dedication. This is SOCIALISM! This is NOT what we want to teach our future generations.
5. Mario Kart Wii does not turn families against each other - Mario Kart Wii on team mode allows families to play coopertively and not hurt each other at all. This brings teamwork, peace, and happiness. NSMBWii, on the other hand, causes frusteration, because there is always "that kid" this can refer to two types of "that kid" A) The smartass that loves to kill everyone by popping them at horrible times or picking them up and throwing them into lava. Everyone turns against him with rage and causes tention. B) The significantly worse player who slows the group down by taking their sweet time, allowing the clock to slowly tick away as agonizing moments pass that limits the time to finish the level before character annihilation. And when they try to make the jump, THEY FAIL! They do this 5 whole times within the first half of the level. Out of boredom afterwords, s/he begins to piss everyone off by pressing random buttons and blowing that antagonizing horn over and over, until other family members playing have so much rage they want to strangle her and bury her corpse in the center of the earth, where the lava will slowly burn his/her flesh alive. But the only legal way to punish is to kill, but NO! THEY'RE ALREADY DEAD! YOU CAN'T DO JACK SHIT! You just sit there with a twitch and a glare, hoping for dire consequenses for the "slow" one.
In conclusion, not only is Mario Kart better and more fun, but it's safer and keeps your sanity. And I thank you. :)