We're going to be ordering a heavy weight muslindrop on which to paint scenery (finally) - unless anyone can give me a good reason to spend $200 more for canvas. Basic price is $500 for muslin and $700 for canvas. Some have said muslin would be fine.
It suspect the manufacturer is simply meaning their heavier weight product vs lighter weight for little more durability. We use a 200 gram / sq metermuslin for drops and it makes a nice sturdy piece. There are zillions of various muslin's with different yarn sizes and counts. The best bet is to simply ask your dealer to show you samples of each fabric so you can decide which best fits your specific application.
NFR drops will paint better, but don't forget to use an FR additive in your paint.