As far as brands go, the pre-sediment filter, carbon blocks stages DI resin are brand-interchangable as long as the housings are the same size as your filter housings (which are usually the 10" type, though the smaller horizontal housings are also used); if given the option vertical housings are probably preferred.
The RO membranes are brand interchangeable as well but they must have the same GPD rating. Using a membrane with a different GPD rating will require a different type of flow restictor.
So as an example you get an 75-GPD RO/DI unit with the Pre-sediment filter and RO membrane from Company A, the Carbon blocks from Company B, and the DI resin from Company C.
There may be different options for each stage of the RO/DI system. For example some pre-sediment and carbon filter stages are rated for 5 microns, others can filter down to 1 micron or 0.5 micron.
I'm not sure if all RO membranes are equal in terms of quality (e.g., rejection rate %), so manufacturer A's 75gpd RO membrane (rejection rate 90%) would be sub-par to manufacturer B's 75gpd RO membrane (rejection rate 98%). As I understand it: every 2% increase in rejection rate doubles the life of the DI resin. So the membrane with a 90% rejection rate will go through a lot more DI resin a lot faster than a membrane with a 98% rejection rate.
