Most people use the MGOPM straight from the bag vs. building MTS (see Forum sticky). I do not use MGOPM I use MTS.
Consistantcy as mentioned. Basically your paying more for ease of use. I buy TopSoil from HomeDepot for $1.17 for 40#s, .87 per bag on sale. Buy time I'm finished building my MTS I'm sure it costs as much(or More) as the MGOPM per pound. The only real difference is I know exactly what is going in my tank because I made the batch.