are you wanting to know what is better from a chassis stand point?
The s2k had two different engines, not to mention all of the crazy stuff you can do to those motors to make more power. They can do 300 whp NA just swapping around stock honda parts. The rx8 drives like it is underpowered, even with some upgrades. The s2k has a very nimble feel to it.
from a chassis stand point, the s2k, even though it is a roadster, is pretty solid. It is going to be a little more difficult to do a roll cage, unless you don't care about the bars being out everywhere. The Rx8, I've heard mixed reviews. Some say the chassis flexes way too much, others say it is responsive. I've found it just drives like a bigger car.
My 2 cents, i you want to go fast, then why don't you buy a little bit older car, like the rx7, and spend the extra money on building that up.
I think you asked too vague of a question to get a good answer, other than do your research...