I have a couple quads with two blades, and a couple with three blades. I like them both. I think Rcmodelreviews did the best explanation of multiblade props I have seen in a while. here is the video. I think a very popular choice right now is the three blade props. All the stores I use have them readily available along side the two blades. I would love to hear someone who has used one of those new six blades. However, I have tons of the two and three blades and got asked the other day by my wife "how many propellers do I actually need?" So I don't think I will be purchasing any more until my stock diminishes a little.
More prop blades can generally give you more thrust for the same circular dimension. But there is a limitation when it comes to what your motor and esc can handle. More blades results in more load.
Pros to 3 bladed props
more thrust for the same diameter which is great when you run a high power motor on a smaller frame.
Cons to 3 bladed props
more difficult to transport. With 2 bladed props you can just rubber band them together and stuff them in a water bottle pouch. With a 3 bladed prop you need to store them flat.
quad storage width with props on would increase because you can't turn the props parallel to frame.
Less efficient than a 2 bladed prop which can result in reduced flight times when you really push it.
Typically more expensive than a 2 bladed prop.
Personally I like 3 bladed props. I am using the HQ 5X4X3 on my quad. If you're starting out or getting into acrobatics and would likely break a lot of props I would suggest sticking to cheap 2 bladed props for the time being...easier to find and a lot cheaper.
More blades = more rotating mass = slower response to motor speed changes. You may have more overall thrust, but it will be mushy. There is also a dimishing return to adding more and more blades. Most of the reviews I have seen on 4+ blades say they would be good for smooth video, but not rippin B@!!s.