DI = deionization
RO/DI = reverse osmosis then deionize
Both create purified water...
Long run RO/DI is more worth it... becos the DI resin wun get used up as quicky with the RO membrane which rejects part of the water depending on the membrane.. You will have two output, one is brine water and one is purified water if I am not wrong.. you will use the purified water and discard the brine water, or you can use if for mapping floor and etc.. as for the ratio and how long it takes I am not sure, have not used it before...
As for the number of stages is the sediment filter and carbon filter you use..