0 ppm is 0 ppm as stupid as that sounds its the same 0 out of a 4 stage as a 0 from a ten stage although the d/I resin will probably last long on a ten stage unit but in the long run those filters cost you money also ....
I have a puratek system and highly recommend it ,simple and comes with all the bells and whistles
http://www.jlaquatics.com/product/vt...+-+100GPD.html