It really comes down to the type of riding you do. I used to have a ZX9R, wich has the same ergos as the ZZR600, just more power. Now I have a Z750S.
The ZZR would be better for the twisties and has a little more on the top end, but is going to make you hurt on longer rides. The Z750S will be more comfortable on longer rides and is a better choice for commuting. The additional mid range torque means you have to shift a lot less and it also turns great in and out of traffic.
I can't imagine you doing to many twisties unless you ferry your bike over to the big island. Even then, the traffic is so bad you never have a chance to open it up. The one exception might be the drive over the mountains from Hilo to Kona.
Personally, I would get the Z750S as I think it would be easier to ride in the Honolulu traffic. I can say my Z750S is more fun to ride than my ZX9R was and it handles a lot better than you might expect for a standard. My body thanks me for it too.