...but you gain
massively improved 135mm focusing capability. Per
@splitimageview's rangefinder accuracy chart, there's no rangefinder lens an M3 can't focus accurately:
Meanwhile, a typical 0.72x film M starts hitting the usability limit with a 75/1.2, 85/1.5, and 135/4, and cannot really be relied upon for 100/2 and 135/3.5 lenses.
For perspective, a "lowly" IIIf can handle all of those easily; as someone whose typical lens kit is a 50mm, 135mm, and either a 21mm or a 28mm, I see no reason to move from LTM to any (film) M other than an M3. The M2 and M4's only benefits are the 35mm framelines. That's it.
(And before anyone says it: I wear glasses and can't see the 28mm framelines in any 0.72x M, so I'd be using external finders for that, too.)