Why not use both and add some other things? MG nor Mushroom Compost alone will be that great. A good mix would be Mg, Cow Compost, Mushroom Compost, Worm Castings, and Pearlite. Another cheap solution is using Evergreen Cow compost($1.39 at Lowes), Evergreen Potting soil(2.39 at Lowes), Mushroom...