Okay, from the Prima guide, paraphrased to respect the copyright:
Gen Mo'Kai: much more maneuverable in the air and on the ground. Faster running, and they can jump and dodge better in the air. They're allowed less maximum health and their weapons do less damage.
Robots: more health, and insanely better dodging ability. They walk & crouch-walk slower, and can't dodge in the air so well.
Anubans (Egyptians): Insanely good air control, higher jump, but can have less total health, do less damage, don't accelerate as fast, and dodge more poorly on the ground.
Juggernauts: Much more health, but ground speed, jump height, acceleration, and aerial dodging are below average.
Mercenaries: More health, nothing else.
Nightmares: walk & crouch faster, but are slow running and don't jump as high.
Skaarj: Faster running, more maneuverable in the air, jump higher, but have lower damage capability and health.