I own both and both are great, as close as you can get for 1:1 movement with a mouse/keyboard/nunchuck combination on a console.
The XIM3 is just for the 360, but can be used on a PS3 with an additional adapter, but can't use the PS3 smart translators that the XIM Edge can use since the XIM Edge is native compatible for both systems, no extra adapters needed and has smart translates for both systems.
It easy to switch between games on the Edge by customizing keys on the keyboard to whatever game your playing, you just have to remember which key you bound it to since there is no screen.
Mapping buttons is really easy as well on the Edge, just press start & select/back then the PS3/Xbox button on the controller, then bind the buttons to what ever you want.
I've got mine set up with a Razer Naga mouse and the PS3 NAV controller and couldn't be happier.