The professional 50# is almost (the Milo boys are always screwing around with guaranteed analysis for consumer grade, but professional grade N content has been steady for years) always going to be a better buy with grater coverage due to the greater N content.
For now, to a degree. But with the new 6-4-0, both have the same N percentage in the latest batches. That said, I paid $16 a bag for 50 lbs of 6-2-0, and the normal price of the 36-lb bags in my area is at least $13-14 currently. I remember crunching all the numbers, and on a per-N basis, the pro was a better deal, but on a per-P basis, I believe it was the opposite. Averaging the two (N and P), it was a wash.
That said, apparently all Milo, pro and consumer, is going toward the 6-4-0 analysis eventually.