The prospect of FOD ingestion into the r/h pack due to loading baggage etc is minimal.There is a deflector door that extends when the a/c is on the ground to obstruct any debris from entering the ram air duct.This is primarily for use when the a/c is in motion.
There is no prospect of the suction into the duct picking up any pieces from the ground.On the classic 737 the suction is induced by a separate turbofan and on the NG it is by a fan common to the shaft of the ACM.
The answer lies as to whether you wish to warm the cabin or the flight deck.By operating the r/h pack,all the air flows to the cabin,the l/h pack 20% will flow to the flight deck,with the rest going to the cabin.If you're the first one there on a cold day I suggest the left!