Over doing the RO/DI

[Basile]

I'm having trouble choosing a RO/DI unit, its not a money issue its a practicle issue. I see 3-4-5-6-7 stage unit now. And some 7 stage unit are cheaper that than a 4 stage unit. So i'm perplex as what use is a 7 stage can actually provide than a 4 can't. To me its seems a waste of filters or an unecessary expense. Any advice on whats really important and whats not, because i'm lost, lol. Thanks for your time.