I've owned and used versions 3-4-5 Summicron when I shot Leica rangefinders (over a 15 year period). I liked version 5 the best, and continue to shoot the R mount, though I understand the 4th version had the same optical formula. Maybe improved coatings? I didn't like the version 3 as much as the others, but I understand it traded resolution for contrast and was optimized for slides.
Frankly, any of the lenses discussed are good, and you'll be pleased with either. Get the one you can afford. You'll be happy with it. The Zeiss lenses have a different look then Leica. Check out pictures on flickr.
The big fallacy in 'Leica Land' is that general shooters need some objectified best, usually defined as the most expensive, or whatever the experts and their charts say, but there is no best, only differences. Good photography depends on light, then composition and only then glass. You can take great pictures with any of the lenses discussed. They are all lenses of high quality.
Having said that, the one lens I've owned that was magical was the 50mm Summilux ASPH. Not only did it magically make my wallet lighter, but it was potent. Stunning at the wider apertures, but even that great lens couldn't save a picture from bad light and bad composition.
A lot of people actually prefer the older lenses for B&W film photography. My girlfriend just put a roll through a 1938 Ikoflex with a simple three element Triotar, and whow! The images knocked my socks off.