Look at the features, there are only about 3 manufacturers of the actually filters in the world so really all your paying for is the extra features like a TDS
meter or flush kit, the filter housings and hosing is all the same look for a seller that has good instructions and good warranty and support. Bulk reef supply is good, I bought mine from eBay and it was a little cheaper but I had to buy a couple things seperately that some of the BRS filters come with. I bought a 5 stage RO
filter with a 3g tank and faucet for $130, I added another stage for DI
(+$20) , then a dual TDS
meter (+$25) then I bought a better tap to tap into my existing waterline (+$10 at lowes) because I talked to some people about it and they said that those cheap self piercing taps are prone to leak so I bought a nicer metal tap from the hardware store. I'm pleased , so for around $185 I got a little better setup than the BRS filter for the same money, I like having the tank so I have a couple gallons on demand and it fits nicely under my sink.
This is the exact filter setup that I got and it's actually a little cheaper now. Like I said add a $20 deionizing resin cannister and a TDS
meter and you'll have a better setup for less money and it's very easy to install took me about 45 minutes and I'm not really a handy man. And adding that stuff on is easy since like I said all the filters are the same the add on stuff hooks right up.
Here's the DI
stage that you would need to add to that kit that I listed, with just reverse osmosis alone you get about 6-16 TDS
depending on your tap water, add the DI
stage and you get 0tds.
And add a dual online TDS
meter to monitor your filter performance, so you know what water is going in and how well your filters are working when it comes out, and will let you know when it's getting time to change the filters, because If you change them just every year, they may still be good or they may need to be changed every 6 months without a TDS
meter there's no way to know.