Of course this is completely subjective, but I'm going to pretend it's objective and try to convince you.
Keys and aspects are really two versions of the same thing. At the very least, they have something in common deep at their core: They are both tangible mechanics that reward players for something they do in the story; and they particularly reward you for getting yourself in trouble. Both of them act as markers of what players want to see in the game, whether good or bad.
Aspects are good in that they're very open -- there's no limit to the number of good aspects you can dream up, and taking advantage of them to get your bonus is pretty easy. All you have to do is show how your aspect applies to the situation your character is in, pay a Fate point and you get a +2 to your roll. If you're particularly creative, you can even pay a Fate point to make something happen, or to declare a fact about the story or setting and make it suddenly true.
Aspects don't only act as a way to spend Fate points; they're also a way to earn them: the GM will "compel" your aspect by offering you a Fate point to make your character get into trouble in some way related to that aspect. For example if your character has a "Greedy" aspect, and you walk by some possibly booby-trapped treasure in a spooky tomb, the GM can offer you a Fate point if you just agree to reach out and take it. If you say yes, you take the treasure, the Fate point, and also whatever consequences come along with that in the story. If you say no, you have to pay a Fate point to the GM to resist the temptation.
Keys are good in that they're more clearly defined, and more subtly nuanced. My favorite implementation of keys is in Lady Blackbird, where it acts as a sort of bonus and experience system combined. Any time you do something connected with one of your keys, you can get either an experience point or another die back into your pool. If you get into trouble because of your key, you get 2 experience points, 2 pool dice, or one of each. If you turn against your key, you get a whopping 10 experience points and your key is destroyed; but those 10 experience points are enough to buy two more keys if you like, or one key and one more advancement of another sort. Once you've destroyed a key, however, you can never take that key again.
=Why I like keys better=
Keys make take more time to write than aspects, and certainly take more time to describe all the different ways in which they apply to your character and the story, but that extra effort you put into the outset really pays off later on. You don't have to argue so much whether or not the key applies in any given situation because of the extra explanation that comes with it, and you yourself are in control over whether you want to follow a key into some troublesome situation (such as with a key of greed), or whether you want to just skip it.
Aspects, at least to a certain extent, take control of your character away from you and give it to the GM. It's not just the temptation of the extra Fate point that may make you want to take that treasure, it's the cost of one of your own Fate points to not do that thing the GM has asked you to do. Now, many players choose the aspect of Greedy because they want to get into this sort of trouble, so when the GM offers them this kind of Fate point reward, they'll happily take it. These players may also feel perfectly happy about paying a cost to resist their own aspect, because again, they willingly gave up that much control to the GM at the outset when they chose that aspect in the first place. These players will willingly compel themselves, and have fun doing it, even though it's a responsibility first given to the GM. But some players aren't comfortable with compels at all -- they feel as though everything pertaining to their character should be under their control and theirs alone, and the GM doesn't have any right to touch it.
Keys not only avoid this problem, by just letting you choose "2x reward vs. 0 reward," they also give you a big payoff for changing your character completely in a way that your aspects don't. The basic 1 point reward for doing anything related to the key drives the theme of that key into the story, the 2 point reward directs that theme towards more tension and excitement in the story, and the 10 point key buyoff drives your characters to grow and change according to the events, or to turn against such a reward and remain true to their original nature, whichever seems more rewarding for the players telling the story. It has all the advantages of aspects in terms of reinforcing themes and creating tension, plus the added benefit of promoting character change, while at the same time keeping the character completely under the control of its player.