A rotary encoder (Rotary encoder - Wikipedia) is better suited if you wanna know not just the speed but also the direction. If speed is everything you need, reeds are better suited because they are more immune to dirt and the like (rotary encoders often use light for reading).
For getting the speed value usually it's better not to count the pulses but measure the time between two pulses. That way you get a value more frequently and have a higher resolution (typically).