Basically depends where you live and what's the TDS at your tap water, you can design your own system accordingly. not everybody need 4 stage, or not 6 stage is better then 4 stage, and so on. I will try to explain.
a little background, add on to what Justin have said up there, a reverse osmosis (RO) device can reduce 80-90% of your TDS of your input water. depends on the membrane you use, you get different efficiency and different output speed, i.e. GPD. the higher the GPD it makes, usually lower the efficiency you will get. also who makes the membrane have a big impact on the quality too. ppl usually replace the membrane in 3-5 years, or less then 60% of the input.
deionize (DI) purify water through chemical process, which means, the more ion it needs to be removed, the faster it will exhaust, i.e. the faster you need to replace the filter. water coming out from it should read 0, sometime 1 if your TDS meter is a little bit off, anything over you should replace the filter.
the basic system consist of 4 stages,
1, sendiment filter trap large particles
2, carbon filter organic chemicals and most chlorination
3, reverse osmosis trap almost everything else
4, deionize filter trap the smallest ionic particles
6 stages usually adds in a finer sendiment filter and a better carbon filter. sometime ppl add UV filter to it too, but since we are not running a lab, I just think UV filter are unnecessary.
sendiment filter and carbon filter don't usually affect the TDS value, but it helps to filter out the larger particles so the RO membrane have less things to do, thus a longer life span. Same as DI filter, you can basically run your tap water through DI filter directly, but you may need to change the filter out every 2 weeks.
so it all comes down to prices of the filter, as I listed the stages up there, that's basically your price break down for each filter. sediment filter is the cheapest and DI filter is the most expensive
so if you live in a place where TDS value is very low, you dont' really need the RO device, you can run it though sediment, carbon then DI, it should give you some time before you need to replace the filter. but if you live in Davis, where TDS value is from 500-700 ppm, you better run 2 sediment 2 carbon, maybe 2 RO 1 DI, or 1 RO 2 DI to get the TDS down to 0. and you may need to change out the filter frequently also. even worse is, if you live in rural area where you are living off well water, TDS value are not monitored. by the state of california, 500 ppm are recommended, and 1500ppm maximum, so if you are around 1000ppm, then you are on your own buddy. good luck.
