Nilch,
I never owned a clamshell, so take that into consideration here. I have had my 6000 for
just under 2 months. Before that i had a 5600. What I have found with the 6k is that I gradually begin to use more and more of its capabilities as time goes by. I thuoght at first that the usb host was "nifty" and not much more, for example. Yesterday I got built in kernel support for an rs232 usb/serial adapter working... now I will be able to connect gps to my usb cheaply. etc, etc.... \My point is, though, that one tends to increase the demands put on one of these things, and the 6000 is simply a more capable device than a 750. Go with the one that will do more for you..
On the other hand, you can sell your 6000 for way too much $$$ if you do it immediately, I think.
All I can tell you is I like my 6k and couldn't imagine selling mine to buy a 750.