I use the Zeiss Ikon, a fine camera to be sure. I like my Carl Zeiss 50/2 Planar but I don't use it nearly as much as I thought I would when I bought it. I actually use my MS Optical 50/3.5 collapsible a lot because I really have come to appreciate how portable it makes the Zeiss Ikon. If I want really low light capability then I use my Voigtlander Nokton 50/1.1. It is certainly a bigger lens than either the Planar or Sonnar, and it is softer wide open than either of them, but if you are accustomed to packing SLRs around, then it won't seem very big at all.

There are many excellent options in the 50mm field for the M mount. I wouldn't be in such a hurry to lock yourself into a specific lens. You may find it worthwhile to contact Lens Rental to see what options you can rent and try before you make your decision. I know that for myself, the lenses I thought were so important when I started, turned out to be not nearly as important as I thought they were.