I've had both set up at one time, with a little utility I wrote to handle moving the files to their proper places via gui when I wanted to switch between them. I've stopped using CGX altogether though because if you don't know enough about display settings, you can overclock and burn out your PIV. There's nothing in CGX to let you know when you've gone too far, and you can go way too far without knowing it. I know a couple of others who did just that with CGX.
CGX has more features than P96, and is considerably quicker because of superlayers. In P96, when you have a window open using "smart refresh" behind other windows and you move it, the refresh slows to a crawl until the window is slowly redrawn. You can't move your mouse pointer, and the system feels like it's locked up except for the slow refresh going on. In ImageFX, having more than two image buffers open can get downright ugly. CGX doesn't have this problem.
CGX also has screen dragging, which is nice at times. The only drawback is that screen switching with dragging enabled can take longer. (Not nearly as long as Windows though :-D - GTA3 anybody?)
If you know not to overdo it on pixelclock speed, CGX is definitely the way to go. To play it safe, either stick with P96, or make notes of the settings in Picasso96Mode and import them into CGXMode.
