I went from an A1P to a Gen 2 RSV-R (2009 model with Ohlins forks)...although I still have the Kwak as it's fun thrashing a 600 once in a while.hehe
The way the two bikes ride, handle, perform, etc. is like comparing chalk and cheese. The Kwak is more frantic and needs to be worked to get the best out of it, as the Aprilia is more lazy with the way it delivers its extra thrust. The Aprilia chassis is far superior, and when combined with the motor, I generally find myself exiting fast sweepers 20mph faster than on the ZX-6R. The brakes, too, are monsters in comparison, being radial Brembo's clamping bigger discs.
On the Kawasaki, the riding position will feel more sports-tourer after the Aprilia, but I prefer the more extreme riding position of the RSV. Fuel economy does suffer, though; I typically get 30mpg compared to 40mpg before, but it's worth it. Tyre wear is also increased, especially if you ride it properly and drive it hard on exits; this bike thrives when attacking corners.
If you are able to adapt to the difference of a V-twin's performance, you will love it!