diffuser or absorption panels, which would you choose?

Sounds as simple as a football bat.

Is there an iPad app that I can use to evaluate my room? I have a acoustical microphone I can plug into my ipad so that's not a problem.

The sofware you are looking for is called Room EQ Wizard. I dont think it will run on an iPad, but it will run on any computer. Watch the instruction videos because it is not self explanatory but very informative if you use it correctly.

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.

This is a good video on bass traps: