diffuser or absorption panels, which would you choose?

Like most things audio , diffusion is not a simple or cut and dry subject. That’s why folks can go on and on about it lol

You can buy a book about diffusion if you prefer. Lots of folks build quadratic diffusers, as they can be a bit more simple to diy if you go that route.

If you have echo issues, use Corning 703 sheets covered in fabric of choice on the walls on either side of listening space. Never use the black foam used in recording studios.

That’s the wrong approach. You first have to identify the problem before implementing the solution. The first step should be an analysis of the listening room. Acoustic companies offer that service. Best to contact them directly.

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.

REW?