Christopher Chen , Apr 13, 2005; 07:35 a.m.
(1) Zeiss-Opton v. Carl Zeiss 50/1.5 Sonnar: I have both & they're of equivalent optical quality. Some prefer the later Carl Zeiss because they're supposed to have been made w/better quality control, as the Zeiss-Opton were manufactured when the assembly line in Oberkochen was new. On the other side, some prefer the earlier Zeiss-Opton because the Carl Zeiss are more susceptible to separation (I believe they switched to an "improved" adhesive that has proven to be less durable).
(2) 50/1.5 v. 50/2: Both are excellent lenses. IME, the 50/2 is just as good as the 50/1.5 @ f/2, even though the 50/1.5 should theoretically be a bit better. The 50/1.5 is easier to find, @ least in the U.S., so I would lean to the 50/1.5 for that reason alone & get the extra speed (there is no significant size/weight penalty).
I recommend posting your query on the Zeiss Ikon Collectors Group on Yahoo (http://groups.yahoo.com/group/ZICG/), where many of the hardcore Zeiss Ikon collectors & users gather.