I had the 11817 ('69-'79) Cron at the same time as the e43 Lux. I found them to be remarkably similar except for the 1.4 and the 30cm difference in close focus.
At some point I sold the cron for the later tabbed version 11819, which is optically the same as the current version. There is a noticeable improvement in sharpness and contrast in the outer regions.
There was no optical change made in the e46 Lux, all they did was redo the helicoid. So basically the lens focuses closer than the optical designers had intended. Not sure if that's good or bad.
I use the e43 Lux on an M9 and feel that it performs as well as it did with film, that is to say there are no glitches applying it to digital per-se. However I was lucky in that mine focuses correctly on the M9. There may be some sample variation in that respect, so some may need to be sent to Leica for adjustment. That could be true with your older Cron as well.
Someone said to wait until you have the body and then try both lenses and decide. That seems like a sound idea to me.
