An important consideration is what sort of pictures in what sort of lighting you usually take. If you do a lot of hand-held candid shooting in dim available light, a 50mm f/2.8 lens will usually be too slow, and even a 50mm f/2 lens may be marginal. A 50mm f/1.4 lens would offer higher shutter speeds and more of a margin against blur caused by camera shake. On the other hand, if you do most of your shooting in bright daylight at small apertures and high shutter speeds, and do not need a large maximum aperture, then you might consider an f/2 or f/2.8 lens as offering smaller size, lighter weight, and possibly higher image quality and lower cost as well. Personally, I do a fair amount of available-light photography, so speed is an important consideration for me.
The Canon 50mm f/1.4 in LTM is an excellent lens, and usable on M cameras with an LTM to bayonet adapter. I own one, and have used it on a Leica M2 with excellent results. It is not the latest or most exotic lens, and newer designs might outperform it in technical tests, but it takes great pictures. Other 50mm lenses with excellent reputations include the Leica 50mm f/1.4 Summilux, Leica 50mm f/2 Summicron, and Canon 50 mm f/1.8 in LTM, although I have not used any of those. The Summicron has a particularly strong reputation for image quality. The new Leica 50mm f/1.4 ASPH is reputed to be an exceptional lens, but is also considerably more expensive than older used lenses. The 50mm f/2.8 Elmar is doubtless much more affordable, and offers the advantages of recent design and modern coatings, but is on the slow side. In the final analysis, "ya pays yer money an' ya takes yer choice."