Figuring out which type of rowing machine resistance type you should buy is a difficult decision. The four options are Air, Hydraulic Piston, Magnetic, and Water. They all have their pros and cons and I have outline this in detail in my post on rower resistance types.
You can also read my comparison articles to help you decide between different resistance types:
Air Rowers vs. Magnetic Rowers
Water Rowers vs. Air Rowers