mixmkr<snippity>
I will say, I've never walked into a big boy studio and seen rigid fiberglass hanging on the walls or stuffed in corners. (ala Ethan Winer style or his counterparts) The movable gobos were the only place I saw stuff like that.
Here's the deal... there are three tools one has for room treatment - absorption, diffusion, and reflection. All three of these can be part of the room design, or part of the room treatment. If you make them part of the room design you (generally) end up using less of all of them, since the room dimensions and geometry do the lion's share of the work.
Few of us can afford/justify a purpose built room, so we are left with room treatment. That's not a bad thing at all, just is. But it is the reason you don't often see a lot of foam or fiberglass on the control room or studio walls in a "big boy" studio.
Foam vs Fiberglass panels - every material has different absorptive properties. Most manufacturers of absorbers publish their own measurements, and these days the measurements are (mostly) meaningful, although you may need to read the fine print to understand how they made those measurements.
Both work, but both solve different problems. And once you understand which does what you can make an informed purchase, and get the results you want.
Before you make any purchase you need to find the optimal location for the monitors and your ears - it isn't always the most ergonomic, or attractive arrangement, so you may have to make some trade-offs, but that is the best starting point.
My current temporary digs are completely untreated. In fact it's a mess! But I was able to improve, dramatically, both the stereo image and the frequency balance by experimenting with placement. It is NOT ideal, but it works. I am not happy with the bottom octaves, but that turns out to be my choice of monitors, not the room (well, mostly, you know what I mean<G>). New monitors will extend the low end, and I may need to make further adjustments, but I'm anxious to give it a try. (FWIW, current monitors use a 5" driver, new ones will use an 8" driver, based on preliminary - read not in my space - auditions this will help a lot.)
Have fun! Make Music!