If you don't want to spend bigger immediately and you're not sure you want to get into anything heavy, go with the Mr. Beer - it's a fine 2-2.5 gallon fermenter , it can make perfectly drinkable beer, and even if you get into more involved brewing there might be recipes you want in smaller amounts so it's not like you're throwing money away. Two points there, though: either replace the booster with a second can of Mr. Beer LME, or a pound of DME from a brewing supply store. The standard recipe has way too much corn sugar to be of much quality. Also, give at least twice as long as the recipes say to ferment: it might be done in a week, but it will taste better if bottled after 2-4.
That said, if you're willing to kick in a bit more, go with what the others are saying and get a full kit. It's more useful if you stay in the hobby, and probably has better resale value on Craigslist if you don't.