I'd rather take Mario Kart 64 over Mario Kart Wii.
In the Wii version, the item spawning in Grand Prix and in Wi-Fi mode can get very annoying,
point that it become lot more about luck than skill. Remember how dreaded were the CPUs in 150cc and Mirror Cup in Grand Prix? Well, how about being hit repeatedly by chained items, such as blue shell, then red shell, then another red shell, and so on? I'm not kidding. If I bought it then get through of it, I would break my Wii from my frustration! I also personally feel that the VR system became a kind of mess, thanks to the unfair item settings. 12 racers in both mode actually did the dirty job of bad item spawning settings, which didn't incited me to buy this.
Mario Kart 64 obviously lack Wi-Fi, but the item setting in this game make the game lot more skill-wise. CPUs never gets Blue Shell, and they appear less frequently throughout the multiplayer mode. Heck, Super Mario Kart doesn't have the blue shell!