Bottom line up front: have you considered a Panasonic FZ200
? I ask because it has a full frame equivalent 25-600mm lens that also has a constant f2.8 aperture.
To continue in more depth:
at distances of up to 50 meters ... I'm thinking a 75-300 telephoto would be best and don't mind using high ISO (3200 or 6400) to catch people walking at night.
That wouldn't work for me because, at 50 meters, the frame would be about 4 meters high. i.e. a typical person would only fill about half the height. Thus one reason for my suggestion of the FZ200's 600mm equivalent focal length which will obviously get you twice as "close". i.e. a frame about 2m high or close to average person height.
The second reason for suggesting the FZ200 is the f2.8 lens because even at ISO6400, in my experience, when shooting people outdoors at night, I would be underexposed with anything slower. (Remember that, for people walking, motion blur will become an issue with shutter speeds slower than 1/125.) And although with some planning, you could use a slower shutter and wait 'til they stopped walking (to cross the street, for example), you will also be pushing the limits of image stabilization for camera shake with even a 1/125 shutter and a 600mm equivalent focal length. (I'm assuming a tripod is not discrete enough to be used in your situation.)
Having said all of that, if you want to look into a micro four thirds (m4/3) camera, while both Olympus and Panasonic have lenses that zoom to 300mm (75-300mm and 100-300mm respectively) which, with m4/3's 2X crop factor will get you the same 600mm equivalent frame size, both lenses are two stops slower than the FZ200's lens at maximum zoom, so underexposure will be even more of an issue.
Also note that, as I type this, Canon no longer lists the EOS-M on its website
, leading some to suspect that the M-line has been discontinued. (Possibly supplanted by the SL1.)