yeah, you have two entirely different approaches to horsepower here. high revs versues displacement. pick your weapon of choice. and seeing that the S2000 engine is already tuned out of its gore, it'll be more difficult to get much more power out of it with comparable mods done to the 350z. you see my point?
i've driven both to some extent, and i think the S2000 is the funner car to drive between the two. but yeah, the 350z certainly felt like the faster car. i like the 350z interior better, it's more useful and roomier. i like the honda's seats better. i would also choose the honda 6 speed gearbox over the nissan's. that thing is slick as all hell.
i think the honda's design will age better than the nissan, but for the moment i like the look of the 350z.
in the end, i would choose the 350z.