In some areas of the country, rockwool/mineral wool is cheaper than rigid fiberglass.
For broadband absorbers, either will work. 6" of 3lb fiberglass 4" of 6lb fiberglass, or 4" of 8lb mineral wool straddling the corner will all do a good job.
For reflection panels, I wouldn't go any more than the 6lb or you run the risk of 'grazing' high frequencies at high angles of incidence. If you're going to DIY them, then 3lb (703 or equivalent) fiberglass or 4.5lb mineral wool is cheaper and will do just as good a job.
Bryan