Here we go.
So here are a couple of cards that are rated back-to-back at Bestiaire.org, a favorite low-tech draft simulator site of mine. Gore-House Chainwalker is the 15th overall common, and Rakdos Shred-Freak is 16th overall. Both cards are 2/1 creatures, and both have a converted mana cost of 2. And, really, they're both cards that fit into the same mentality: aggressive, Rakdos decks. So is there really that big a difference between them?
First, let's dispense with the differences in mana cost. Two hybrid black/red manas is roughly the same as the 1R cost as far as mana availability. They're both very red cards and belong in red decks, so you're most likely going to have the mana you need for either of them. So neither one is noticeably more or less of a mana problem than the other.
So, this really comes down to a battle between Haste and Unleash, two keywords that again fit into aggressive decks. But Unleash is miles more useful here. The way I like to think of it is this: either way, you're creating what amounts to a repeatable burn spell. You're going to attack most turns with these two guys. Either your opponent will decline to block and you'll get in for beats, or they'll block and try to defray the damage, or destroy your creature, or both.
So, you can either have a 3/2 creature bashing away, dealing 3 damage to the face, or challenging a defending player to trade his centaur token, or his Guildmage, or his Vassal Soul. Or, you have a 2/1 that gets to attack a round earlier, but can be disposed of by all of those 3/3 centaur tokens without a second thought, or blocked without repercussion by Axebane Guardian, Concordia Pegasus, or a leashed Dead Reveler.
In most red decks, and in Rakdos decks in particular, the goal is to put pressure on your opponent early and often. I feel like nine times out of ten, you put a lot more pressure on your opponent with a 3/2 attacker than with a 2/1 hasty guy.
Joe's pick: Gore-House Chainwalker