I also was puzzled about which Muslin to buy. Since we live in a rural area and Wal-Mart is our only fabric outlet, I just bought the heaviest one last time I bought.
I use Muslin for foundation piecing, and sometimes in patterns. They have two grades and the more expensive is probably better. However, I just bought the cheapest one; $1.95.yd. The other is $2.00+ a yard.
I love to have it on hand, as it does make a good foundation.
Good luck!
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