So...not to evade the OP's question, the simple answer to what would be best in your room would require more "discovery". But what would be most predictable and across the board most helpful in your room size: 2-4 absorption panels being 2'x4'x3" (min.) placed halfway between speakers and listening position.
That being said, I do like diffusion at this reflection point under certain conditions...the midrange (+/- 800 Hz) can open up and sound great.
In the second room with the vault, in that type room to really improve the space it will take a fairly aggressive combination of broadband absorption and diffusion...obviously this immediately speaks of some strategy involved with the treatment. It may very well be that "as-is" is fine. It's very doubtful that just a couple panels will make it better.
Hope these suggestions offer more assistance
