Making a small wedding cake - just two layers. Top is 6 " - would you do an 8 or 9" bottom?
AI normally do 6 & 8 together. But I am only a hobbyists :)
For just a two-tiered cake I'd do a 6-9, or a 5-8. If it's a shorter cake like that I personally think the 3" difference looks better. When you get into a three-tiered cake I do 6-8-10 all the time, but a 6-8 looks kind of squatty to me. A lot of it depends on the design, too. These are 5-8 http://www.acaketoremember.com/images/1378600_10151667540958671_1185784198_n.jpg
There's not a whole lot of difference, really, it's subtle, but the top one being a little narrower makes it look a little less top-heavy.