MG soil has a lot of nutes in it already. One of the biggest mistakes people make is add more nutes to a soil with premixed nutrients. IF (this always starts the debate) you're going to use MG soil...I say don't touch it with nutes at all. You'll end up flushing the soil in a panic when your babies start to get crispy.
Fox Farm soil is a completely different deal. It doesn't have a NPK number that I know of. It's filled with good stuff like worm castings, bat guano, etc. It has a ton of great micro nutrients, absorbs a ton of water & is nice & fluffy.
If I had a choice between a $5 bag of MG or a $20 bag of FF or ProMix BX, I'd go with the last 2 every time.
My 2 cents!
Sheepdog