Pros on Alakazam: He just looks so awesome. He has that long 'stache, and those two spoons....he's just the cats.
And he has a beastly special attack, with great speed to go along with it. In the games, he's used by one of the coolest gym leaders--Sabrina
Cons on Alakazam: He has poor defenses, and if a Pokemon tank with high Sp. Def appears, he's a goner. Easily. You have to trade him to make him evolve, which is a hassle or even an impossibility for some people. He's slow to level up, and it takes him forever to learn another move besides Teleport (unless you have a TM)
Pros on Gardevoir: IT has a graceful design, and nice coloring. It looks feminine (although it can be a boy too, but meh). Its defenses are better than Alakazam's, and it doesn't need to be traded so that it can evolve. It also doesn't have a bad Special Attack, at all. It can learn lots of good attacks. It's a solid Pokemon.
Cons on Gardevoir: It looks like a girl, no matter what. It takes forever to level, especially since it has the same Teleport problem that Alakazam has. It's not as fast as Alakazam, so it may be at a disadvantage in that way some of the time.
So...after weighing these things, it's actually pretty close. So, I'm gonna have to throw in a little extra. Back in Generation 1, almost nothing could take out a good psychic type, and Alakazam was at the top of the list. (2nd only to Mewtwo, really) Gardevoir wasn't introduced until Gen. 3, and by then the power of the psychic type had diminished very greatly. This means that Gardevoir is by no means the powerhouse it could have been if it had been introduced back then. So, because of his awesomeness back in the day, I'm going to have to vote Alakazam by a very slim margin.