Chazz,
This is one of those 'if you ask 5 people, you'll get 5 different answers' questions...
However, FWIW, I give you my opinion....
Currently, I have the new SL-C3000 Zaurus. I really, really like it and enjoy using it a lot!
It's probably the best little device I've had and I've had lots of them!
Going all the way back to several Newtons, Palms/Clies/Handsprings, HPC's, and PPC's...
Prior to my new Z, I've had a C760 and C860. Both are excellent units.
Now for the editorial opinion....
Personally, I really like the design and form factor of the clamshell units. They just feel right to me. Sitting the Z on a desk and using it as a mini-laptop works great and is the way I use it most of the time. The keyboard works surprisingly well for this.
Sometimes, I use it in tablet or PDA mode. The controls work well for this too, even for one-handed operation.
The point is that I feel that I have the choice of how I want to use it, in what mode, either mini-laptop or tablet style.
I don't think the SL-5500/5600/6000's really give you the same level of flexibility. What you do get is a lower price, which may or may not be a controlling factor for you.
If budget is the driver, then I'd go for the 6000, if not get one of the clamshell series.
Just my 2 cents....
HTH,
Mark