I'd go for the S2000. The RX-8 has a known appetite for oil due to its rotary engine. You may love the rotaries, and I'll admit they are a great engineering concept, but you won't love keeping a couple quarts of oil with you in the trunk whenever you decide to take a road trip. Nor will you like Mazda's corporate explanation for the problem, should you try to take it back for warranty repairs: "They all do that."
I actually like how the RX-8 looks, but I'd be far more comfortable with the S2000 from a reliability point of view. Of course, given the ability to insert a third option here, I'd wholeheartedly recommend the Mazda MX-5 instead-- the original Japanese ripoff of the old-fashioned English roadster, with a much larger performance aftermarket to support your need to modify it to your heart's content.
