Well, the decision is up to you. You said already that you are tight on budget. 15 inch rims and 15 inch tires are your best bet. There's nothing wrong with 15's as they perform just as well as 16's. 16's will definately be a bit more expensive in both wheels and tires.
15's will be lighter in weight and on your wallet. Mate the Rota wheels with 205/50/15 tires and you got an excellent performing wheel/tire combo.