Having the C860, I can concur that the display is not great in bright sunlight. How often is this an issue for you? It is not for me.
Otherwise, I cannot imagine why anyone would purchase the 6000. The clamshell form factor is infinitely superior in my opinion.
The keyboard is another major consideration. There is no comparison. The C860 is superior.
The 6000 has builtin wireless, and this was a mjor issue for me. It took me a long time to decide that the form factor was much more important to me than the wireless being built in. Your priority may differ.
Finally, as of today, the pdaXrom only runs on the SL-Cxxx series. And in my humble opinion, X11 is the one thing that makes the Zaurus trump every other PDA/MobileTool on the market today. When you see Firefox render a webpage perfectly, when you compile GNUChess and it just runs, when you use a REAL mail client, when you change your window manager each week, then you will realize that this is what the Zaurus was meant for. IMO of course.
Difficult decision.
For what it is worth, I paid $200 more for my C860 than a 6000 would have cost, and I would make the same decision again in a heartbeat.