The low and mid-range frequency response is similarly flat for both mics and both mics are characterised by the same tight hypercardioid polar pattern. BUT>>>>> The high end frequency response is quite different! The Revox has a distinct presence peak at around 5khz whereas the Beyer has a gentle lift at around 10khz. (This is due to minor design differences in electronic components. )
In practical terms the difference is this :- Although these 2 mics have a very similar general character, and are often used for the same tasks, the Beyer M201 is rather more accurate whereas the Revox M3500 appears to be somewhat brighter (which may sometimes be heard as an advantage!) Either way both of these microphones are superb perfomers, especially on drums and percussion.
So why did Revox alter the HF response for the M3500? In the days of analogue tape recording, when using machines with no noise reduction facilities, it was common practice for engineers to add 3 or 4 db of boost to the incoming signal at around 4Khz whilst recording and cut by exactly the same amount during playback This had the useful effect of reducing tape hiss by 3 or 4 db whilst maintaining the flat frequency response of the recording. Cunning these old engineers! (This is of course roughly how Dolby B works, but done manually!)
The Revox M3500 was intended for use with Revox 2 track recorders such as the A77 and B77. These machines did not supply phantom power and hence the microphone needed to be a high quality dynamic. The Beyer M201 was an obvious choice, with its wide, flat frequency response and ability to handle rapid transients. Also, these machines were not blessed with any sort of built-in noise reduction or EQ facility. By redesigning the HF response of the M3500 with a boost at 4-5Khz it is my guess that Revox were simply aiming to create additional brightness in the region of tape hiss that could be reduced during playback, thus improving the signal-to-noise ratio of their tape machine.
P.S. I like the M3500 so much I just bought another one!