Some tips about the reverse osmosis system
Does Reverse osmosis water good for you?
Countertop RO or Under sink reverse osmosis?
Reverse Osmosis with Tank or Tankless?
The tank offers ease of use with automatic filling and a water reserve is always available.
On the direct flow side without a tank, limited risk in return for lower pressure and the need to be patient. Except to multiply larger membranes (mounted in parallel). To increase the cost the loading of a membrane not always be immediate.
The right compromise could be the small tank or the tank under constant dynamization, the safest approach.