It depends on what you like. If you can handle the 335 weight, then go for it. If not, then go for the 320. Personally, I find that the extra mass of the 335 really helps the stability of the racquet on defensive shots.
You say the 315 is too light, yet you're considering the 320, which is only 5 grams heavier. That's like the weight of one thin overgrip plus a rubber band dampener. It seems to me that if you consider the 315 "too light", then you would feel better with the 335.