<p>I have seen mechanical (micro switch), optical  and magnetic(magnet + hall sensor) end stops.</p>

<p>Are there any differences in how exact they switch at the right location? If so which are the most precise?</p>

I think there are several factors involved in which sensors are best, but the general ordering for me would be Hall, Optical followed by mechanical. All of the types will be subject to some drift due to vibrations and changes in the printer over usage. Therefore it's the ease of adjustment as well as the accuracy of the stop which counts in the assessment.
In my experience the hall effect sensors are the most accurate and easiest. They don't rely on physical switching (as with the mechanical) meaning there is no "wear and tear" on the component and the point of switching will remain fixed. They have a potentiometer which can be adjusted to make the position of the stop change without any mechanical intervention allowing very fine tuning. They can be very accurate.
Optical are similarly accurate but usually have a fixed component that cuts the beam to switch on/off the sensor. The adjustment of the stop will usually be mechanical because the mount points will need to be adjusted - this reduces their accuracy. There are various adjustable mounts to alleviate this on thingyverse or the like.
Mechanical switches are similar to optical in terms of adjustment with the added inaccuracy of the actual switch mechanism which may degrade over time.