diffuser or absorption panels, which would you choose?

As already suggested, watch the YT videos mentioned.

I used a combination of absorption and diffusion, alternating panels, and keeping 1-2 feet between them. I went with only absorption panels on the front wall and diffusion/absorption on the back wall.  Alternating diffusion and absorption on the side walls.   Bass traps in the upper room corners.  None on the ceiling. It’s made a huge difference in my 12’ x 20’ x 8’ room. Used only for home theater. There is much better clarity in movies, better dialogue, much tighter bass and overall a more immersive experience.

If you were only to do one thing to a room it would be big bass absorbers on each side wall, cover the wall with them. Not the corners, the side walls. Getting bass under control is job 1, always.

Or Room EQ Wizzard...

I'll take the bait. I would use only absorption.

Why? Because I can control how much I use.

I would treat the front 1/3 of the room first to deal with first reflection issues. Then I would proceed with caution.

Having said that, I don't use specialized products. I use furniture and wall hangings and thick Persian rugs with heavy felt underlayment.

Has anyone listened to a room with the old concrete acoustic ceilings? They were the first gen version before the softer asbestos version came out it the late 60s and 70s. I had a house built in the late 50s and no room has sounded as good from a clarity and bass perspective.

My thinking is that bass goes right through and is reflected back with softer absorptive/diffuser materials but that the concrete material has a small but noticable effect on all frequencies.

Ceilings with this material cannot be scraped. The whole ceiling needs to be cut out...that's why they're still around. Flipping a house with this ceiling is not cheap.