Well I think my first point would have to be of course the Sigmalux will have limited functionality on your FM3a, but will positively sing on your F6. The Sigma of course, has no aperture ring. I have some experience with the Milvus, it's a nice lens, but very large. I can't make an argument that MF will be bad with the ZF, but I can tell you that manually focusing the Sigma is fine. Similar to other pro spec AF lenses with built in focus motors. The Sigma tho, can be completely focus optimized using Sigma's USB dock. You'll need a digital camera for confirmation to do this, but you can do it. I have with my 24 & 35mm ART lenses, which were both way off, and now they work great! The focus throw on the art is shorter than the ZF but still an AF lens. I can't say whether you'll be satisfied with it, I was personally.
For my buck I'd say get the Siggy. It's gotten to the point now where the Zeiss and the Siggy will have different IQ attributes here and there in subtle ways. People will see what they wanna see in that regard. They frankly aren't different enough to spot within a blind test. I'd want to take advantage of that great F6 AF personally. If you want that classic Zeiss look, I'd tell you to find a used Planar or Makro Planar for your FM3a. The Planar exudes character and AFAIK the Makro-Planar wasn't even updated for it's Milvus edition so that'll tell you how good it is. And they won't dwarf the FM3a. Those old ZF/ZF2 lenses look great on the FM/FE bodies.