diffuser or absorption panels, which would you choose?

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.

This is a good video on bass traps:

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.

I've never seen concrete ceilings? Is this for a bomb shelter? 

I have a basement with a cement finished wall which have a rough surface and Nice acoustically but the basement ceiling is so damn low it's a pointless exercise. But the ceilings in these basements tend to be fiberboard tiles versus plaster and this is quite nice actually. But still the ceilings are so damn low.