I own both, an S2k (MY07) and a 2006 350z Base (soon to be a 370), so I can give you comparison based on that. To me both cars are fun to drive, but in a different way. The S2k is a point-and-shoot car, very precise with very fast reflexes. It's also a lot more raw than the 350z. Switching from driving the S2k to the Z it feels a lot bigger and heavier, but there is also a lot more power. Where the S2k is all about finesse, the Z feels more brute-force, almost like a muscle car compared to the S. The ride in the Z is also a lot softer. After you spend a lot of time in the S and get used to the ride, switching to the Z feels like you're in a Cadillac- well, that's a little bit of an exaggeration, but not much... Can't comment on the interior since mine is a 350z (I hate my interior and all of the peeling rubberized paint...). Overall, I really enjoy both cars but prefer the Z because it's brutish personality matches mine
although occasionally I get a craving to blast around in the S. On the other hand, my wife prefers the S, she says the Z is too big and soft for her taste.