I think full ring can give better rakeback also, than 6 max, if I have it figured right. I think the sites make more off rake (when subtracting rakeback from their profit) at 6 max than they do at full ring. I think that's why they switched from 10 max full ring to 9 max full ring, and why 6 max tables are promoted so much. Under the old system, if everyone at the table had max rakeback, the sites were hardly making anything at all from the rake, maybe even losing money when rakeback was sometimes actually higher than the total rake collected.
Also it depends on your style. LAGs probably prefer 6 max.