There are 8 characters to choose from. They fit into 3 different categories: Heavy, Middle, and Light weights.
Light Weights
Peach, Toad, and Yoshi are the lightweights. Most people prefer the lightweights, they are definitely the best for time trials.
Pros: They have the best acceleration. They don't lose much speed in grass or sand. And they have a higher maximum speed than anybody.
Cons: They lose speed when turning corners without sliding. And they can be bounced around a lot by the heavyweights, even the middleweights sometimes.
Middle Weights
Mario and Luigi are the middleweights. I've read that they are good to pick if you are a beginner but personally I never use them. These two are average racers and don't have any real advantages or disadvantages. They can bump lightweight but can be bumped by heavyweight.
Heavy Weights
Donkey Kong, Wario, and Bowser are the heavyweights. Supposedly these guys are the hardest to use but I don't think so. These guys are mine and my friends favorites. I'm always Wario.
Pros:They don't lose much speed if you turn a corner without sliding. They can bump other karts around quite easily.
Cons:They have poor acceleration. They lose a lot of speed when driving in sand or grass.
