Quick take
A certified pitcher is the cheapest option. Countertop RO makes sense when broader contaminant reduction matters and the room has enough counter space and power.
Lowest-cost option: pitcher
Pitchers require no installation, no electricity and almost no setup. Their contaminant scope depends on the exact cartridge.
Best broad-treatment option: countertop RO
A refillable countertop RO unit can move between dorms and apartments while avoiding permanent plumbing.
Space matters
Large feed tanks and hot/cold countertop appliances may be impractical in a small shared room.