Well, like most Allied SP guns, there don't seem to be many of these in 28mm scale, those that are seem to be USA only and Resin.
I could be wrong, but I think that when you are considering things like the M10/M18 which are already widely available, you are missing out on rare kits like the M7 which would most likely have less competition.
Of course, someone, somewhere might be working on plastic Priests, but they are hiding the info well if they are
I'd choose your Sherman over others (If PSC ever get it back in stock) due to functionality of the 3 weapon options, I'd choose your Priest if you were to make one, because no one else makes them