I'd keep Biggio, because he's a proven veteran. If you needed someone as a backup right now, I'd grab Rivas. Thurston is unproven right now, so it will be difficult to predict what he will be. Maybe once Thurston has landed the leadoff or #2 spot everyday, then jump on him. Another option would be Bobby Hill, if he's available in your league. He should be their leadoff guy everyday this year.