40 or 50 refers to the amount of force it takes to draw the bow...more force=more power behind the arrow= farther shooting distance, more power to the hit. 40 pounds versus 50 pounds. It doesn't sound like alot but if you've never fired a bow...go with the lower poundage...unless you are a big guy then go with the 50 pound bow. Realize that you have to pull and hold while aiming.
You get arrows based on your arm length, not the bow. Put a tape measure at the center of your chest and measure out to the tip of your middle finger. Add two inches. That's how long your arrow's shaft should be. You can go longer than that measurement but definitely not shorter. The closer you get to the measurement the better.
My Jaguar is a 50lb bow. It requires concentration of upper body strength. I find that the higher the trajectory I shoot then the less control I have over the shot. However, I am not a frequent or avid archer. I purchased the bow specifically for the possibility of hunting large game. (States usually require a minimum 45# bow.) It is backup to my firearms.