If you are flat grinding, it really doesn't matter.
I like to hollow grind, and use an 8" wheel. A 10" wheel would be "less" hollow ground if you are looking for a compromise.
A 6" would give an extreme hollow grind and would mainly be useful for thicker blades that are short from edge to spine.
While you may not flat grind now, you may in the future. And it helps to match the wheel on your grinder to the wheel on your buffer.
So...if you are given the choice, I would personally choose the 8" wheel.
Dan