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Red, this one is easy. Hands down, the 50MP and Nikon, provided you can deal with camera stillness - MUP, etc.
Technically (i.e. macro/micro contrast, detail definition, MTF) it's better at f2 but neither of these are great there, to be expected. By f4 the 50MP is a veritable powerhouse, many believe it is the sharpest 50mm made, I am of this view also, until the 55mm Distagon steals the crown. It's very well behaved out to 15mm IH.
It is also a beautifully clinical lens, and has much less distortion than the M equivalent you mention, around half the level of distortion in fact. It is the only ZF lens I would find a use for (Contax guy), and it's quite versatile, fine for objects as well as landscapes. Bokeh is personal but you see many great examples from the MP.
Overall its best aperture performance is around that of the 21mm Distagon. These can both crunch with the best of them ;-) It's the kind of lens large format shooters treasure, if I can put it that way. Your 50/3.5 may not seem so phenomenal if you get one.
It is such a good lens a lot of people dislike it. For reasons that remain a mystery despite their arguments. I believe some lenses are held to higher standards than others. What type of film do you have in mind might matter, this is a very high contrast lens...more difficulty with tonal control unless B&W.