5.1.2 vs 7.1.2 vs 9.1.2 depends on your room size and number of rows of seats.
My room is 26x22, and I went with 5.2.4.
I think most rooms are about 14x16 or 16x18 and 5.1.2 or 5.1.4 is probably great.
Some guys think that having as many speakers as possible is better, but I don't think so.