During the prototyping phase, we are forced to use a very rare type of propeller (6 to 7 inch) which limits our selection but once we move into phase 2 where we can start reducing the frame's size, we can finally move into the 5" propeller category which I presume will be our final product size. This final frame will use the Gemfan's 6-blade propeller. It suffers slightly in efficiency but noise and turbulence is greatly reduced.
Since each propeller blade is essentially a rotating airplane wing, more blades means less volume of air is forced down with each blade rotation. Less volume directly means less vibration, and ultimately less noise which we want for an indoor aircraft. The downside (if you can call it that) is that the aircraft itself stops sounding like a drone and starts sounding more like a jet turbine. Some people actually prefer this so it's a matter of taste really.
Propellers are usually rated by their diameter and pitch. While the diameter is self-explanatory, the pitch rating can define how the aircraft moves. Higher pitch rating usually provides more maximum speed of the aircraft but also more turbulence and stress to the motors, whereas lower pitched propellers provide more torque but less maximum speed, less noise and consume less power.
Indications are usually in imperial and the first number always indicates the diameter, followed by the pitch. In example, a propeller can be marked as 6x4 which means it will be 6 inches in diameter, and in theory the aircraft will move forward 4 inches with each revolution of the propeller. Some manufacturers mark their propellers as 6040 which is essentially the same as 6x4.