ML 50s--Which to keep?

I look at it this way, Noel.  You're not going to know until you shoot them.  You'll be using an adapter to use them on a digital camera, so that creates another variable that no one can predict how it will come out.  And what they render on film will not necessarily be the same on a CMOS sensor.  Then you have manufacturing variables from one lens to another, even in the same production run.

One is just slightly faster than the other, so there is only a small difference in the DOF at wide open.  I think you would see a bigger image quality difference if you were comparing a 1.4 with the 1.9.  They are both good lenses, and I'm happy with my 1.9.

PF

ridgeblue makes a very important point concerning the impact of using these lenses on a CMOS sensor. I have the ML 55 f1.2, 50 f1.4, 50 f1.7, 50 f1.9, 50 f1.9C and 50 f2 as well as their Zeiss equivalents and the ML 1.7 and 1.9 are truly excellent and under-appreciated lenses. For out and out sharpness, from f4 to f11, the 1.7 has a slight edge but it's only noticeable at the edge of the frame - they are nearly as good as the Planar 1.7 at the centre. Wide open, the 1.9 has the advantage (at least on my copies) at the edge of frame and has a more regular bokeh, the 1.7's being more like that of an old Biotar lens.

While I prefer the 1.7, I can't make a case for it over the 1.9 (over the 1.9 C - definitely) - probably for DCSC or DSLR users, the lighter weight and shallower depth of the 1.9 might give it the edge. But as ridgeblue said, you need to shoot with them both and then decide. Let us know...

I probably will, now that I've tried them. I compared them directly:

f/1.9:
Palmetto, Wekiva River by nrparsons, on Flickr

f/1.7:
Palmetto, Wekiva River by nrparsons, on Flickr

And then I used both of them for a variety of other shots, and I'm pleased with both:

f/1.9:
Pay Here by nrparsons, on Flickr

f/1.7:
Canoes only by nrparsons, on Flickr