agree with bob. to really feel the effects of the c3 a tune is needed. the dual cai is better bang for the buck.
having said all that i wouldnt do either first!!
the most important mods you can do at the beginning is cooling mods. supercooler upgraded i/c pump 180 tstat, -10 * fan mod and separating the system will ensure any future mods perform to their peak expectations.
all the cooling mods cost approx 650$ and while it provides no hp inc. it does ensure the power you have will be there when you want it!