We currently are using pro-mix with the mycrorize in it...makes life grand...We have used canna soil for a long time, FF, and Sun 4...currently we actually have sun 4 and pro mix side by side (extra soil leftover)
In our experience, sun 4 and promix grow virtually the same, the pro-mix has a high porosity bag that helps the plant suck up more nutrients allowing for bigger crop over all.....sun 4 seems to stay wet longer with slightly worse drainage...
About 8 5 gal pots out of a bail of sun 4 and 10-11 5 gal pots out of promix (29$ a bail)
So, seems it prety much becomes a question of if you want to not water as often or if you'd rather have your plant suck up more nuts....