What temp is the water coming out of the ground?
Here in Alaska the water temp is 35F and while many will say I am wrong I will use a softener and have been for 20 years on water that will have up to 20ppm and get away with it as long as the iron is clear.. ie the test for the iron goes to say 10ppm and stays there it does not increase to 12ppm+.
As for the difference in resin, I have been using standard resin in the units around here with no problems.
Might I ask why the SST-60? it is good, more spendy for a little extra in capacity....