Definitely Koviu, although I don't think it's by a mile. Defensively they're about the same, speed is about the same but offensively Koviu is just a better player. Kesler does bring a certain grit factor, and maybe by the end of the season the offensive discrepancy will be a bit smaller.
I'm a huge fan of Kesler, but have to give the nod to Koivu. He's a complete hockey player that can be used in any situation. Kesler has improved his offensive play, but there he still handles the puck like a grenade when on the powerplay.