Everyone is different, but for me, quads don't hurt any more or less than anywhere else. For delts, I have to go for the rear head, which seems to hurt less the next day. If I hit the front head of dead in the center of the delt, it burns! But when I confronted the nurse who wanted to give me my flu shot in the front head of the delt, she said I was the opposite of most. She did as I asked, and gave it in the rear head, and it was fine. She was surprised.
Moral of the story is, (using the known "safe" injection sites), try pinning various places and find the spots that are best for you.
Charles