50 series means 5", 45 is the pitch. You can use any pitch, the higher it is the more aggressive the acceleration. Not sure exactly how it affects top speed, I only have a basic understanding.
Well I got a little bit better understanding. But I wouldn't say that I'm an authority.
The more blades ( or the more aggressive pitch ) will move more air. This has several different affects on flight. The more air you move, the more current the motor has to use in order to try and make it's speed. Probably resulting in lower flight times too. Also, the more blades you have on one prop, the more drag will be introduced into the airstream. Sacrificing top end speed. So, there are several things to balance to get maximum performance. And they are all related to one another. IE: Number of blades, Pitch of the prop, Motor Speed (RPM), Motor Torque and Motor Current. Now, how do you combine all that to make it work?
I usually just pick the color that looks best.
Somebody more intelligent than me may come along and make some corrections to my statement.