"big breasts or flat chests" is a false dichotomy!
i like slender girls with B- or C-cup, like what i usually see in real life here. there should be some shape to them, not totally flat (i've only ever been with one girl like that and she was korean, oddly), but not big enough that they move too much.
i love aino kishi's body, for example:
i'm not into flat chests and it's not that i hate big boobs per se, more that they don't tend to be on the body types i like best. but after reading this thread i guess a lot of people consider the above flat, so i'm gonna vote for that!