Dave,
You will see a ton of threads on here debating the Ebay units against the Airwaterice.com Typhoon III unit... don't get sucked in...
The Typhoon III is a good unit-- has a 98% rejection RO membrane (DOW Filmtec) (removes 98% of everything) vs 96% for the latest Water General with the GE Desel membrane (this is at 50 PSI where the Filmtec achieves its 98%-- the GE Desel can achieve higher rejection if you can raise the pressure up higher.
I've posted this in several threads but everyone just kind of ignores it but it is a simple fact...
The worse your raw water is, and the more water you use, the better off you are with the Typhoon III... over the long run it will cost less to run because your DI resin will last longer.... but if you don't use a large amount of water and/or your raw water is not that bad... then it might take several years to recoup the added cost of the Typhoon III... if your water is bad and you use alot, you could recoup the extra cost in as little as 6 to 12 months...
the water general one isn't bad, just not as good as the Typhoon III.....
The RO stages job is to clean the water as much as possible before it gets to the DI stage..... the DI stage takes whats left and through an ION exchange removes any remaining dissolved solids...
For instance... my unit (a SPectrapure)
My raw water is 411 PPM TDS (I have well water)
My RO output is 8 ppm (98% reduction)
My DI output is 0 TDS....
If my RO membrane was a 96% rejection (reduction) then my DI resin would last 1/2 as long
If a 90% rejection, then 1/5th as long....
So as you can see the quality of the membrane does matter but like I mentioned above... it can take a while to recoup the cost of the better unit-- it is totally dependent upon how bad your water is and the amount you use... You should still be able to achieve 0 TDS with either unit, just the cost to operate them (replace resin is the main cost) is different.
Hope you found this informational...
Spuds/Bill