Pressure reverse osmosis systems force water through filters to remove large contaminants and harmful chemicals and then through a membrane to remove very small contaminants. These systems are effective in removing 90% of metals like copper, chromium and lead and minerals like sodium chloride, as well as, fluorides and nitrates.
A reverse osmosis system is almost double the cost of the under-the-sink filtering product. However, it is extremely effective in removing contaminants, particularly arsenic. If you live in an area serviced by private well water, you might sleep a little easier with this system.
Reverse osmosis systems are large, and can take up all the cabinet space beneath your sink. Like the under-the-sink filters, they require another faucet for water dispensing and can be very slow. These systems require good water pressure to operate at an adequate output. More than five times as much water is wasted as is filtered with reverse osmosis. If you are looking for a speedier filtering than the drip of the carafe, you may not get it here.