They do different things don't they? Good Mornings strengthen the back and the glutes and hamstrings whereas power cleans let you train your explosiveness and moving the bar faster.
I'm moving onto a split in a week or two, and my assistance for squats is going to be Good Mornings, not power cleans, because I want to strengthen my back more for deadlifts ad it always seems to be my weak point. Also, from what I've seen, more people have talked about/acknowledged the carryover from GMs to DLs than the carryover from PCs to DLs.
edit: sorry, I thought you were talking about good mornings...