So Aquamax is the best feed to use what is the differance between the 500 and the 600? Why would I use the 600 over the 500 or vice versa??

I called the feed store for a bag of this Aquamax feed and it is a whopping $23.00 a bag?? they also carry something called Southern states feed which is 36% protien which is $12.75 per bag...Is it really worth the extra $10.00 per bag??? If it is I WILL SWITCH TO THIS AQUAMAX 500 or 600!!