If you want the purest water possible, then you should get a unit with both as RO and a deionizing module. My choice of units was purchased for several reason.
1. I wanted it to be portable so i could bring it with me to fish weekend where I was selling fish either as a vendor or a room seller.
2. I wanted it to make as pure water as possible, so I chose one that had bot an RO and a DI (deionizing) module.
3. I found ones where I could chose to have it come with and input that fit the standard garden hose connection.
4. I did not need high output so I opted for between 50 and 100 gallons/day.
So I got a 75 gpd, 3 stage {carbon-->RO-->DI}, portable unit. I replaced the RO and carbon modules once. When I finally needed to replace all the modules I opted to buy a new unit. However, I really wanted a sediment filter in front of the carbon. So I now have a 4 stage unit.
It is light weight, easy to use. I attach it to my utility sink which has a fauet that accepts garden hose connections and provide a way to deal with the waste water. I batch into a 20 gal can. From there I fill a series of storage containers which last me about 4 weeks I have a couple of TDS pens and I can test the output which is usually 0 TDS.
In theory, my RO/DI water should be 7.0 ph. But this is on;y the case in a vacuum. In the can it is exposed to the air which includes CO2. When that gets into the water it creates a small amount of carbonic acid which doesn't last long but is also constantly replaced. So the effect is that acid causes the pH of the water to drop below 7.00.
Here is the nice part of this. My first 3 stage unit about 10 years ago cost me $120 plus shipping. The 4 stage was bought in the past 6 months and cost me $159 + shipping.
My main use of the above system at home is to maintain a 55 gal tank at a pH of about 6.0 and TDS no higher than in the 60s. My tap is 7.0 and TDS that can range over years from about 80 to 115 ppm. it depends on the aquafer and rainfall. I have seen it as low as 53 ppm after a few weeks of very heavy rain storms.
Here is where I buy:
https://store.afwfilters.com/brands/afwfilters.html
Click
Categories, then
Reverse Osmosis Systems. My next click was not
Aquarium RO DI Systems, it was
Portable RO Systems. My first system was the Mikro Omega 3 Stage Portable Aquarium RO/DI System with 75 GPD Membrane. It is now $139 = shipping. The one I replaced it with is the Mikro Delta 4-Stage Portable RO System PLUS DI with 75 GPD Membrane (Former Psi system). Both offer:
2 Connection Options
Your choice of connection options for quick and easy setup
Garden Hose Connector - Simple connector fits standard garden hose thread
Diverter Valve - Connection to most standard faucets with valve to direct water to system