Much better than those foam tiles, which are expensive and only address high frequencies (which are usually only a minor problem) is building your own acoustic treatment from rockwool and wood.
Make a wooden frame - a couple of inches thick (or more if you like). The bigger the better, but if you want them to be moveable around 6ft tall by 2ft wide is pretty good. Back the frame with plywood and fill it with high density rockwool. The higher the density the better, but even just standard insulation rockwool will have better effect than the foam tiles.
Make a few of these, perhaps fashion some kind of base for them. For maximum effect there should ideally be an 'air gap' between the panels and the wall. This will increase their attenutation of lower frequencies.
To get an idea of what Im describing, watch this vid:
http://ronansrecordingshow.com/2012/10/mobile-acoustic-panels-from-la-sound-panels/The good thing about these (or bad, depending on the room) is they will warm up the room a lot.
Of course, if you dont want them to be mobile just make it like a dry wall type thing; but with the air gap and the high density rockwool
In choosing which room to record in, Id go for the one with either:
- most space, mainly for practical purposes (sitting or standing comfortably with a guitar, and having space for equipment and treatment.
Or if they all have the same space, the one with highest cielings