Re: RX8 or SR20DET 240SX?
You are comparing a old cheap car, to a newer, more expensive car. Most people try to get the most for the money they have, so if you are comfortable spending as much as you would on a rx8, you shouldn't even be looking at a 240sx.
I might be bias, but I'd go for the s2k, or even look for a 350z, maybe a wrx.