whats the difference

Approximately 50% more blades on the 3-bladed prop

A 2-bladed prop is usually a little bit more efficient than 3. It's also easier to manufacture which means it will be cheaper to buy. And it is easier to balance a 2-bladed prop.
A three bladed prop will be smaller in diameter compared to the 2-bladed prop it replaces. For example, 12x6 two bladed prop might give the same performance as an 11x6 three bladed prop. This smaller diameter means that the tips of the prop are moving at a slower speed. This reduced tip speed will make the 3-bladed prop quieter than the 2-bladed prop

Advantages of 2 blades
Cheaper
More efficient
Easier to balance

Advantages of 3 blades
Quieter