In walls are doubly tricky in some ways...it is going to depend greatly on their position relative to side walls, type of wall, etc.Are you using these for ambient type listening where you may be walking around or not sitting in a sweet spot?
You say it's open, not really a closed room. In that case I'd tend towards the 8" in wall just to move a bit more air volume. Can help project the bass farther.
Though if it's more ambient, no single seating position, use in such a space, four 6.5" spaced around may actually be better than two 8" not sure if that's feasible.
You can also consider looking into an inwall or ceiling subwoofer, or a wireless sub, to round out the bass with 6.5. You may find you will want to even with the 8"s. Again, it kind of depends on more specifics.