You are correct in your assumptions. The GSR is not much if at all slower. Depends really on the driver. With equal drivers the RSX-S will be slightly faster. You will save yourself money on buying the GSR, money you'd be able to if you wanted throw right into mods. Insurance may be slightly less on the GSR but probably not a noticable difference. The GSR can look very very good and tasteful, but that goes to say for both.

Since you are posting this in the RSX forum I'm sure you're ready to hear biased opinions, and so is mine. I personally thing the RSX looks better from factory than the GSR, but both can be made to look incredibly good in their own respects and attributes.
The RSX offers the new k-series engine, which by the way is the wave of the future as far as Hondas go. It responds amazingly well to bolt-ons, and has .2L more displacement equaling a noticeably torquier engine and throttle response.

You asked for opinions and so you have it. My $.02
It comes down to your personaly goals and plans for the chassis. Let us know what went down and oh...

welcome to the boards.