I have both. The Nexio is better is terms of readability, speed and PIM. The keyboard is not nearly as good as the C760 (READ: It is horrible). It is removable though. The Nexio (except ofr the keyboard) is a very nicely designed PDA. You will not be running PocketPC apps and many WinCE apps dont work because of .NET compat. issues, so realize that you will be stuck in the world of finding compat. apps. Also, battery life sux!!!!
The C760 is smaller. It is fast also, but Qtopia has its drawback. The software is easier to find and there are alternative OS\'s for it. It doesn\'t have the integrated WiFi or Bluetooth that the Nexio has, but I find the battery life to be much better even with a CF card in. Also, it has CF and SD expansion, where the Nexio only has CF. Screen is smaller, but its more compact design will let you carry it in your pocket. You will be hard pressed to do that with the Nexio unless you want people to stare.
I have some Zaurii, as well as a Sigmarion III and Nexio.
If you are a WinCE person, go with the Nexio. It is not a bad HPC. I use my C760 more often.
LD