I did a Google Books search as well, but I think the higher hit count for "quadrotor" is an algorithmic anomaly. The number on the first page is an estimate; the one on the last page is more accurate. If you go to the last page of hits for each term, what do you find? Usually the numbers will adjust somewhat. I found that "quadrotor" actually ended up lower than "quadcopter", despite initially showing thousands. Are you able to reproduce that effect?