BillO:
justjed:
typically, in tach applications, I see hall sensors used.Well, I'm not sure where you have looked, but this is generally not the case. I see them used where magnets are employed for other reasons, but the by far the most common approach is optical. Optical is cheap, has huge bandwidth, and is easy to implement. You might want to consider it.
I think it depends on situation. For outdoors environment where infrared is plenty (from the sun) and dust and dirt are unavoidable, magnetic sensing is best fit, like for car speed. For indoor well-controlled environment lots of infrared optical sensors are used, like old-school computer mouse with a big ball inside, most rotary encoders for precision motion stages, laser light choppers etc.