Not good at explaining but here goes...
It doesn't matter which one you use because you will still end up with a 6 1/2 inch block. BUT I would use the 2 1/2 inch square that Marcia shows because you are using 2 1/2 inch strips in your 9 patch and the seams will/should match up. Do a test snowball and 9 patch and put them side by side and you'll see what I mean.