75gpd or the new one from buckeye is the best the higher volume per day membranes loose rejection rate and thus waste more di resin. get a good air tight storage container and make big batches it better for your filter run long batches instead of short runs. i have a 55g barrel that only gets to half way before i top it off.

shane if your psi into the membrane is 65psi+ then running a dual 75 would better than one 150gpd