Stretch and go for a turbo, you will never miss the extra $$ in the long run. Or you can pick up great buys on a fresh, pre-owned one. There's nothing like the boost, and you can always play with chips for even more fun.

If you cannot get the turbo, get the C4S. I have owned lots of street race Porsches (904, Carrera RS 3.0, for example), and you will quickly want more comfort for daily use and some occasional track fun...