I have a 4" Bearcat I use on a 29 HP tractor, and it occasionally loads down with a tough log. The self-feeding works great as long as you keep the blades sharp.
It seems that manufacturers rate the size of their units differently - my 4" Bearcat takes considerabley larger logs than my previous unit of the same size.
I think a blower that sends the chips to a pile is essential if you plan on doing much work. Otherwise, you end up spending a lot of time raking chips.
Finally, pay attention to how difficult it is to get to the blades to sharpen them. They need it pretty often, and some units require a lot of disassembly to get to them. My Bearcat requires removal of one bolt than the unit swings open to allow access to the blades.
It seems that manufacturers rate the size of their units differently - my 4" Bearcat takes considerabley larger logs than my previous unit of the same size.
I think a blower that sends the chips to a pile is essential if you plan on doing much work. Otherwise, you end up spending a lot of time raking chips.
Finally, pay attention to how difficult it is to get to the blades to sharpen them. They need it pretty often, and some units require a lot of disassembly to get to them. My Bearcat requires removal of one bolt than the unit swings open to allow access to the blades.