You can use either size, but the 8 inch would look better because you will have more room on the sides. I have stacked an 8 inch on top of a 10 inch and it looked fine.
How many tiers are you planning total? If you are only having two tiers, I think at least a 4" difference would look better (8" round). But if you plan on doing 3 tiers total, you could do a 6 - 9 - 12 or something like that.
