Lucas wrote it so, Lucas get's final say. Lucas wins. End of argument and I'm okay with that.
However, it makes more sense to me, knowing Han's character, for Han to allow Greedo to shoot first. At that point in the story Han was not one of the good guys. But, that does not mean that Han was a cold blooded killer. Hans one flaw is his ego and ego would have dictated that he controlled the situation to where it looked like the other guy shot first. Also, Han was in a bar where Greedo would have had known associates and if Han had shot first they would have killed him in a heartbeat and Han knew that. Greedo shot first, or Han made it appear that way, and then Han takes Greedo out.
Bottom line is that if Lucas says it happened a certain way then that's how he wanted it to happen. Any other arguments contrary to that is moot.