Corn snake hands down. Both reach adult size within a year under normal conditions. Garters are fast, fragile (to handle) and aggressive. Go with a corn.
Youll also have a massive variety of colors to choose from with corns. With garters you have only albinos of a few species and thats about it.
Definately go with a corn. I bought a little baby corn about 11 months ago and it is now about 26 - 28 inches eating full size frozen mice. The snake is cool man real easy to handle and no trouble at all. I would recommend going to cornsnakes.net to check out all the different colors before you just buy one - you may see a color variation you like more later so check them all out prior to purchase.
Okay, then corn snake it is! I can't wait until friday. But, how many hours a day do I leave the heat lamp on?
EDIT: One more question. I used to have a tortoise, and it had a plastic bowl that is designed to hold water. Do I need to clean this with soap to reuse it?
I would recommend 3 - 4 hours a day. I would also recommend a heat pad under the cage on the same side of the cage as the heat lamp that stays on. The goal is to give the cage a warm side and a cooler side so the snake can regulate it's own body temperature.