I bought a Z, so I guess that answers that question. When I was looking I didn't even really consider the S2k. I think they're entirely different cars, and one who likes the Z won't like the S2k and vice versa. To me, the Z is...
1)
Sexy. The S2k is cool/"cute" depending on how you look at it. I look at it like I look at a Miata. I like the car, but it never really hits a high point for me aesthetically. The Z does have some odd characteristics though, namely the backend. I don't care for it.
2) Power. Having driven both the Z, the S2k, and others in the same price to power range, the Z gave me the best feeling of power and control. I feel as connected in the Z as I do in E36 and older 3-series. The S2k has a lot of handling advantages though I believe.
3) Coupe. I don't like roadsters. I believe most destroy the aesthetic of a sports car.
4) Heritage. The Z heritage appealed to me to some degree.
The S2k has a lot of advantages too though:
1) Roadster. This is a huge plus for a lot of people obviously, but not me.
2) Honda. Honda delivers a better product than Nissan. I've had 1 Honda and 1 Acura over the past decade, and I loved both.
3) More responsive handling. The Z can sometimes feel bloated. The S2k isn't as tossable as a Miata, but it feels more capable than the Z in this respect. The numbers may disagree, but this was my impression in driving them.
4) Less boy racer than a Z I think. I think the S2k has a slightly more mature presence, but this might just be my impression.
In the end, it came down to how I felt when I looked at the Z and when I sat in it. Both are great cars as far as I'm concerned, and you can't go wrong with a Honda product.