I bought an 860 new and a week later the 3000 was announced... then the 1000 and then the 3100. I was a bit sick.
I also own a 6000, and love the built-in wireless but the USB host give huge scope for enhancement.
I find I swap cards a lot with the 860 between wifi bluetooth and big compact flash. If I could get hold of a 3100 with built-in bluetooth it'd be the perfect device.
If you buy the 860, you should assume that it will be a stop-gap measure. If you are not sure about owning a Z, it's a cheap way of finding out. If you are very serious about your PDAs and linux, you'll regret buying an 860 only to sell it in 6 months to upgrade. However, if Sharp produce a newer model, you'll be pleased you bought the 860 as an interim device rather than spending the extra on a 3100 and then wanting to upgrade again.
BTW, the 760 is functionally identical to the 860, just a different case style.
Tough choice. Maybe you just have to accept that there's no right answer and get the device which will do everything you want to do at this moment.