I'm asking myself this exact same question...I like both but for very different reasons. Once I've figured this out for myself I"ll let you know!
1. I like the Nismo and especially the additional power and suspension tweaks...I don't like the lack of a Nav system/bluetooth/XM (at least I'm not expecting them on the Nismo)...it's also just a touch too "boy racer" for my tasts.
However, I'm still seriously considering one.
2. I love a drop-top...nothing quite like moving down the road with the top down on a beautiful day and I love the additional ammenties on the roadster. I don't like the fact that Nissan seems to believe that their roadsters shoudlnt' have a rear deck spoiler so I'd likely add one.
I wish the roadster had at least the extra ponies of the Nismo even if not the tighter suspension if only to compensate for the additional weight.
It's a delema to be sure!