I love the s2k and it was a very tough choice before I went for the Z. personally I think the manic redline style of driving the s2k made driving more of a hoot but the z was just more of an overall package. Both had the performance but the z looked and felt better inside, and looks a hell of a lot better, modern and more aggressive with the exterior styling. Also if your going from an s2k to z then the z feels a lot heavier, but the fact it has so much more torque kinda makes up for that.
I finally made my mind up when I was test driving a s2k and saw a Z, instantly I knew I would prefer to be in that car.