Hickory is denser than say the oak we can get here in N. America. The oak from Japan must be different than what we get here, dunno, seems different to me.
The hickory is nice because it isn't so brittle, however it's a little softer, so it dents, and raises splinters around the dent area easily. Hence it requires more sanding in my experience.
I make my own bokken, so the advantage that hickory (hickory heart, or impact grade hickory) has over oak is that they hickory I can get is more suitable to impact weapons than the oak I can get. I think it prettier too. Some of the pieces of white oak I've found are down right ugly. Not that this matters in practice of course, but if I'm spending hours shaping a bokken, and getting the hairy-eyeball from the wife, I want a good looking piece of wood.
The bokken I'm using now is maple, it's a little brittle for impact, but I've laminated two pieces together lengthwise, so hopefully that will help. It will definatley splinter less as it is much harder than the hickory. My last hickory bokken I had to retire: too many splinters and chips for safety. Didn't break though!