Since I don't have online on my Wii yet, I went to a friend's house to do some Wi-Fi matches with SSBB and MKWii. :)
In my opinion, Mario Kart Wii has the better online of the two games. You know who you're playing against, as well as what country they're from, which is pretty cool. I even got to race against some Japanese players. :D
I played A LOT of online races on Mario Kart, and I had a lot of fun. I won quite a few races, too. I'm definitely looking forward to playing more online races on MY Wii.
As for Brawl... that's a different story. I played a couple "With Anyone" matches, and the lag was pretty bad. One of my matches even completely FROZE before it dc'ed me! It got pretty annoying after a while. :(
The "With Friends" matches have MUCH less lag. I watched my friend play a couple matches with someone on her friend roster, and those matches looked a lot better than the Anyone matches. I didn't do anything in Spectator Mode, though.
Geez, I need to get Wii Wi-Fi soon. T_T