Stirring the cinders of an old thread...
I have both devices. As a basic PDA, the C860 wins on account of software. Cacko 1.22a is a great
ROM. It's compatible, beautiful and everything just works. The clamshell form factor is a winner in my book too.
The 6000 has the most beautiful display I have ever seen on a handheld. It is bigger, and has slightly higher resolution than the C860. The keyboard is layed out similarly to the SL5500's, but with a right-hand shift key in addition to the left hand one and with better action than the rather chintzy one the 5500 has. The built in Wifi is a big advantage for me. I have a 4GB microdrive stuck into the freed up CF slot, so the device gets closer than anything I've owned to a laptop replacement.
The 6000 software scene is beginning to pick up, but there is still a better selection of both basic OS and application software for the C7xx and c860s. I end up using the 6k more nowadays because I'm bouncing between the various new ROM choices testing them out and plotting how I'll fill up the microdrive.
I've posted elsewhere that my ideal device (for today) would be a clamshell Z with the 6K display, built in 802.11g and
a built in microdrive. What I really want is Debian/arm, Qtopia, GPE and OPIE apps to work seamlesly on my device, whatever the hardware. Neither of these things are available today. I am patient.