I think you really need the hard top with an S2000 there is no lining on the roof on the inside so it looks a bit unfinished. The hard top solves this makes it a lot better. I have friends that have had them and they were really impressed with the lining in the Z4 roof they always say they forget it's a convertible with the roof up.
Still great cars if you ask me lots of fun to drive just very differant to the Z4. I also like the simplicity of the styling I think it's helped them age really well.