Hi Naru,
In general, all the Ro/DI's waste about the same amount of water. Some are more efficient if they have a booster pump etc installed on them, other then that very little difference in that regard.
Spectrapure CSP and 5 stage units we believe are the best for overall quality of water output though. The highest purity. Then Kent Marine and Spectrapure MPDI are close 2nds. Our newest lineup, Coralife seems pretty good as well and a lot less expensive.
I would recommend Spectrapure CSP or 5 stage if you want the absolute best. If you want to save some money, i would look at the Kent or Coralife then.
thanks