From MarioWiki:
Just as in Mario Kart Wii, each character gets their own specific stat boost that accounts for kart customization, based on their weight class. The units are out of 6.[2] In general, light characters have high values of Acceleration, Handling, and Off-Road, while heavy characters have high values of Speed and Weight. Hover over the weight column to see the characters in the represented weight class.
Here are the different characters unlocked from the beginning and their weight classes:
Feather:
Toad, Koopa Troops
Light:
Peach, Yoshi,
Medium:
Mario, Luigi
Cruiser:
Donkey Kong
Heavy:
Bowser
Unlockable characters:
Feather:
Shy Guy, Lakitu
Light:
Daisy
Medium:
Mii
Cruiser:
Rosalina, Wiggler
Heavy:
Wario, Metal Mario, Honey Queen
Source
If your air speed is equal to another speed, it's usually better to stay in the air. This is because you can cut hills or turns that you can't while on land or water. For example, take this crude diagram:
Here, Player A (red) jumps off and glides far, landing farther up on the hill. Player B (blue) jumps off and dives quickly, landing lower on the hill. Player A landed farther up the hill, having a large advantage over Player B.
A good example of a situation of this is the first jump in Rock Rock Mountain. Just after the jump, there's a hill that slowly goes up. It's better to glide as far as possible to land as far up the hill as possible.
Also, this holds true in situations like this:
(That blue... thing is supposed to be a flight ramp.)
Here, the grass is cuttable by flying over it, rather than just hitting the ground.
Even with these things, sometimes taking the land route is still the better option. For example, flying through a walled canyon with turns is pointless, since there no flight advantage, and it's better to get the mini-turbo boosts from ground travel.
To sum it up: generally, the shortest route is the fastest, unless there's a very large difference between land/air/water stats.
Best Answer
This is your ranking. When you complete a cup, you get a ranking based on your points and your time (and possibly other factors such as how many times you fell off or hit opponents with items). The best ranking is three stars, followed by two stars, one star, A, B, C, D, and finally E (I believe). The best ranking you've ever achieved in a cup is recorded for each cc, so if you redo the 50cc Mushroom Cup and get three stars, that will replace the one star.
Getting higher rankings isn't required to do anything in the game, but once you gain rankings of one star or better for every cup on every cc, the game will display one star alongside your name in races. Same for two and three stars.