The 50's choices are getting bewildering - to put it mildly. They are all very good - there are some minor differences in how they render our world when you shoot.
DR Summicron 50f2: Nice lens, very smooth rendition. Without the goggles slightly limited close focussing - with goggles you can get really close. Nice match for a M3 too. Look for the later #, above 1.900 000 as they did upgrade the coating slightly and improved the contrast.
Rigid Summicron version I: nice lens too, but can be a bit "flat" in contrast and the "infernal" lock can drive you to distraction. Can be dismantled though and the "catch" on the lens ground down.
ZM Planar 50f2 - better than the Summicrons, medium contrast, good close focussing and nice out-of-focus rendition. I call it the Summicron-killer. Good prices too. Need the hood as a bumper.
Zeiss C Sonnar 50mm f1.5: Disregard the hype re focus shift (all lenses have focus shift.) Gorgeous black/white lens - smooth as silk with great shadow separation too. It is my "standard" 50 these days, both on my M's and on my Nikon Rf's.
If you need good wide open sharpness, dont disregard the Nokton 50f1.5 - just because it is less expensive that the others it is by no means inferior in quality. One of the better lenses @ f1.5. Contrast is a bit higher than the non asphericals, but not high enough to cause problem in the darkroom.
Of course, all of these pale in comparison with the 50f1.4 Asph Summilux - but you can almost buy all of the others for the money you would have to spend for it. very "aggressive" contrast - almost looks "pixillated" in the out-of-focus areas. The only real competition for this lens is the Millenium Nikkor 50f1.4 and to some extent the C Sonnar 50f1.5.
I dont have enough experience to warrant an opinion on the Hexanon 50f2. From what I have seen, it looks good and the users tend to swear by it.
Have fun deciding. I am envious of the One Lens/One Camera. OK, in the words of JR Tolkien "One Lens To Rule Them All"!
