Since the 1.4 update made both biomes available in a single world, there are a lot less reasons to choose one over the other. For example, if you only choose crimson for the ichor, you may as well take corruption at world creation since you can just make crimson in hardmode when ichor becomes available. There are still a few things to consider, such as biome mimics (the biome mimic summoned by the key of night depends on which biome you chose durning worldgen, which makes the other one harder to farm). With that said, here is how I see things:
Melee - crimson. Honestly, corruption isn't that much behind in terms of melee loot, but the crimson does have some slight advantages, such as crimson armor in pre-hm, the flesh knuckles and fetid baghnakhs being easier to aquire, and the vampire knives.
Ranger - corruption. Musket is better than undertaker early on; demon arrows, while being hard to farm consistently, can occasionally be obtained from devourer and EoC kills, slightly reducing the grind for jester arrows. Having corruption as your natural world evil also means an easier access to putrid scent, and as such to stalker quiver and recon scope.
Mage - corruption. Vilethorn needs no explanation, but the mana band is also nice to have early on when you can't stock up on mana regen potions. And whilst life drain is pretty great, the utility of the clinger staff cannot be underestimated either.
Summoner - corruption. Pre-hm, crimson offers absolutely nothing to summoner other than the brain of confusion, which isn't that much better than worm scarf to justify choosing the crimson; aside from that, corruption also has wrath potions, which can boost minion damage unlike rage potions that do nothing for summoner at any point of the game. The random chance to get ancient shadow armor pieces early on is also nice, as they can boost whip speed pre-bosses. And no, I didn't forget flesh knuckles - they are pretty useless compared to some of the other accessories you could be using, and the only reason you'd wanna get them is because you're forced to use the feral claws due to lack of autoswing on whips, "so I might as well upgrade them for 8 extra defence". In reality, the bonuses provided from flesh knuckles can be achieved by simply drinking ale and iron skin potions (+1 warding accessory/bast statue if you are picky about having equal defence), and you would have an extra accessory slot for something more useful.
I am personally a sucker for corruption due to the darker theme and since it ties in better with the overall setting of the game, but the crimson also has it's place. It's ultimately up to personal preference, as the bonuses provided by either biome in pre-hm are ultimately negligible in the long run.