I, of course, highly disagree with Matt's evaluation of SMK vs. MK64. The control and gameplay is much more solid in SMK, but we've been over this in other threads.
I feel like the age-old debate hinges on what game you grew up with. Mario Kart 64 played a much bigger role in my youthful multiplayer-playing days, while Super Mario Kart was never a huge focus.
You'd think so but SMK was my first and my family and I played a ton of it. But when MK64 released we switched right over to it and never looked back. No longer did we need to swap controllers. I went back recently and played SMK. It's super floaty controls are hard to re-adapt to after so long but it can still be fun. In the end I look back more fondly on the N64 one.