which ro unit to get?

[canadawest]

I wont speak for Darren, but according to the manufacturer of my RO/DI unit they recommend changing the sediment and carbon pre-filters every 6 months (about $20 for both) which should allow the RO membrane to function well for about 2 years (about $75 for a replacement)

If you also have the DI cartridge, it is supposed to process 400-500 gallons before needing replacing (about $30)

Of course this all depends on how poor or good your tap water is to begin with. If it is high in TDS (total disolved solids) your pre-filters will exhaust a bit sooner, if your tap water is already low in TDS then your filters will all last that much longer.

Best to own a TDS meter to check the TDS levels of both the processed effluent as well as the waste water from the RO membrane. This will tell you exactly when it's time to replace the cartridges.