depends on your needs, size of tank, hof often you change the water, if you have a holding tank because an RO unit is no good unless on a constant auto water changer system or has a holding tank.
I got a 25gpd unit and a 100 gpd unit.
Going to take me 2 days to fill my 210 with the 100gpd unit as opposed to 8 days with my 25gpd so I had to upgrade.
You get what you pay for, I have 2 spectrapure RO units and love them