You'll find the rangefinder patch of a Leica M much improved over an S3.
In the past, about 10-15 years ago, I used a pair of M5's, and at another time had an original Nikon S3 for a summer. I always thought that the M5 was the best metered film M followed closely by the M7. It's a very pragmatic, intuitive camera. A sensitive meter and easy to read/use match/needle layout. It's one of those function over form beasts, because it is bigger than the traditional M. The rangefinder patches of the several M5's I've had my hands have all been good. Only 0.72x viewfinder mag but easy to see 35-50-90 lines. One consistent of all the Leica's that have passed through the house over the last 25 years (a shameful number) is that the contrast and clarity of the rangefinder patch have all been very good; the same, no variations.
Nikons aren't like that. Small, light, with the gorgeous 1.0x viewfinder, but the rangefinder patches are all over the place. The S2's range generally from good to excellent, but the S3 I had was very faint, even after I cleaned all the rangefinder mirrors.
Not sure I would've kept the M5's in the long run, given their size, and because I now prefer meter-less cameras, but the only reason I sold them back then was because I was at the time getting out of rangefinders. I sold the S3, however, because of the lack of contrast of the rangefinder patch. In the Nikon lineup I prefer the S2, solely because of rangefinder patch issues.
I now shoot an M2 for a rangefinder.