They are both metal !!!
Point 1
For some reason the B8 bearings seem to run smoother than the newer models, but I would say it's a personal choice as to which one does more for you visually.
Point 2
A lot of the parts for the B8 are no longer available, but you can find B8s on ebay to salvage parts off of.
Point 3
The B10 is pretty much the same as the current floor polisher / carpet shampooer so there would be more parts available for it.