This is where those types of questions should go.
First though, a little generic advice:
How should we know?
You're the one who knows what you find fun to play, not us. A lot of the time people try to remedy this by saying "who should I buy Vlad or Ezreal. I play Morde and Sion." This only makes it harder to answer, because on one hand you're saying "he already plays two beefy guys, maybe he wants to try something new" and on the other hand you're saying "he only feels comfortable with beefier champs, so Vlad".
The only thing we can really give less subjective (but still very far from objective) advice on is which champions are currently considered more powerful - but this should be a complete non-factor in your decision since
- you should play who you have fun with and
- power is fleeting.
If you have to ask...
...you probably shouldn't buy either one of them. Feeling that you don't know which is the better choice implies one or more of the following:
- you don't understand the game very well yet
- you haven't tried the champions in question ever before
- you tried them and didn't know what to make of them (because you don't understand the game very well yet)
If one or more of these is true, you shouldn't buy either champion yet. Wait until they're free and play some games with them. See how you like them. Would you buy an expensive pair of shoes without even trying them on and seeing if they fit?
If you're still at this stage, you should focus mostly on buying runes and practicing with the champions you do own and those that are free.
Fun and what matters to you
If you find one choice more fun but think the other is "better", get the first. You having a good time and wanting to practice will easily close the gap between the relative "power levels" of any champions. Don't feel that you shouldn't buy X just because Y exists. Just do what you want to do.
Edited by The Hound, 23 March 2012 - 01:44 AM.