The only real advantage a turbine with two blades is the reduced cost and weight of a third blade. However, since a two blade wind turbine can’t capture as much energy from the wind as a three blade turbine can, it has to be designed to operate at higher RPMs to produce the same amount of power. when it comes to small wind, this can hardly be seen as an advantage!
They also suffer from a dynamic inbalance. For instance, when the top blade is in the wind the bottom blade is being shaded by the tower. This causes problems with yawing and puts unnecessary wear on the bearings. This makes them particularly unsuitable for high wind areas.
Three blades successfully eliminate these problems. In the end three blades rule the day. However, a turbine with two blades may be suitable for you but that depends on a few more factors.
Photos Courtesy of DOE/NREL.