First the objective technical, then the subjective personal, assessments.
The Canon 50/1.4 is very capable and probably the best objectively overall. The later black barrel 50/1.8 is known to suffer hazing problems that may result in permanently etched glass - make sure you're getting one that hasn't suffered this fate, or get the earlier chrome design. Filters are oddly sized (40 and 48mm). The 50/1.4 is rather large on a lot of LTM bodies, but perhaps not surprisingly balances pretty well on the large late Canons.
The Summitar is a little softer wide open than the Canons, and probably has a little more distortion (I don't particularly care about minor distortion, so I haven't paid much attention on this point). The OOF areas may be slightly less pleasant with the 6-blade aperture, most notably in points of light, which will exhibit a clear hexagonal shape; there are no click-stops. It uses an oddball filter arrangement, but this can be dealt with by use of an adapter. Note that the Summitar likely will not collapse into the Canon body due to the position of the light baffles.
Subjectively, I find the Summitar a lens with great character, while the Canons are technically better but a bit less interesting to me. I am willing to accept the faults of the Summitar (clickless aperture, fragility of the glass, optical limitations at large openings) because I love the way it renders, especially around f/4. I use mine on Barnacks primarily, so I am able to collapse it.