Perhaps I need to take both 6" AND 8"... but I already have to lug so much gear around!
Now I think that would be insanity. Just pick one system and stick with it.
I originally went with the 8" to help cure the 12-24mm softness because its right at the bleeding edge for close focus without a diopter. The 10-17mm doesn't have these issues. FE lenses work very well inside domes. If all I had was a 10-17mm for WA and I wasn't into split levels I'd go with the 6".
If you are a nut like me and want to travel with all your lenses, then the 8" makes some sense.
Advantages:
-Actually lighter if you need to use:
-Macro: 60mm, 105mm VR w and w/o diopters w and w/o focus
-WA: 10.5mm, 12-24mm, 15mm
-Better corners/focus with rectilinear WA zooms
-Better split levels
Cons:
- More expensive
- Bulkier if you have less lenses
- More O-rings and some difficult assembly (its a bugger to disassemble these requiring a strap wrench or 2)