diffuser or absorption panels, which would you choose?

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?

Check around your area to see if you can find a local builder. I found a guy via FaceBook (which I never use for such things) and he built panels and bass traps for a fraction of the cost of some of the ones you'll see offered. Granted, there's nothing wrong with those services, room correction, etc. Please don't hear me dismiss that. But I was able to build, add, and add a few more, finally with 2 diffusers for several $100 vs. thousands. And custom made, same insulation options, and I was able to get the precise sizes for my room.