You will have a good time in any program but I would also vote for the open wheelers for the reasons stated. Plus, there is a certain raciness to the single seater you don't get even in a race prepped MX-5.
I would strongly recommend buying some proper racing shoes (unless yours are real race shoes), they make a big difference for not a lot of dough, very important to have good pedal feel.
Go fast, have fun
