Is there any real difference between 320 and 350 CCs?

I had my consultation with my PS and was stuck between 320 and 350 CCs. I really want them to look natural & proportional to my figure, rather than wanting to have huge boobs (Full B is the goal). I did like the 350 I think I'm just so hesitant because I've never had boobs, so everything looks big to me. Also, I understand the procedure but to decrease the chance of rippling (I'd be getting saline unders), would the PS overfill a 300 CC implant? How does the overfilling process work?