The difference for me is when I'm on my back I'm able to hold him close and he's able to see my face and I'm able to see his (if I can open my eyes lol), whereas, with doggy-style, it feels a lot better and he's able to go deeper, faster and harder without worrying about hurting me. I'm also more vocal when it's from behind because of how much more intense the feeling is when he's thrusting into me. Also gives him the chance to run his fingers over my back and shoulders =D