Also is going to depend on which gear isn't down and your aircraft type (e.g. high or low wing, jet, turboprop, etc). As BOAC said, check the QRH.
Arguments for landing asymmetricly would be at least partial braking and directional control as well as perhaps less demage to undercarrige.
I'd agree with that if it were the nose gear that hadn't come down. Not so sure if it were either main gear.