My opinion is very simple: real is ALWAYS better and, even if it seems ironic talking about girls,
size doesn't matter as long as the shape is natural and harmonic.
Of course I can't talk on behalf of all the guys, but it's sure that nothing arouses me, and I think almost everybody, than a nice natural breast.
My favorite model is Sandra Shine, who IMHO has the best natural breast in the world for size (a large C cup), shape, firmness and nipples, as well as Carlie Banks or Peaches for example.
The problem with fake boobs is that at the end of the story they are... fake and almost always, as Sabrina Deep pointed out, the result is simply awful.
I think that everybody dreams a girl with a perfect breast like the ones listed above, I think nobody dreams to touch boobs totally unnatural or plastic, not me for sure.
I know that many girls go with surgery for self confidence issues: girls don't! It's something we guys don't like and moreover it's also something deeply unhealthy because you're putting in your body something which is not natural.
Oh wait, maybe I actually can talk on behalf of all the guys:
name one thing guys wanna see more than anything else in the whole wide world!