Gas shocks are considered better and are usually more expensive. They use pressurized nitrogen to absorb shock, as opposed to hydraulic which uses an oil. The oil in a hydraulic shock may foam under heavy use and lose efficiency.
Back in the 1940's and earlier you could rebuild the hydraulic shocks in many vehicles but since then they cannot be rebuilt, just replace with a new one.
yes because it does not pollute the air and can help keep the seas from being overflowed from melting ice caps and/or glaciers. it also will slow down the death of the human race, or otherwise known...