I have a 50mm f1.4 AIS and a 50mm f2.0 AI long barrel version. Of the two, the f1.4 makes for a great low light lens, though I prefer using the f2.0 long barrel. You can see one example of that shot wide open in daylight
here. Just for comparison, you can see a
B/W shot under low light with the f1.4 AIS.
I have also used a 50mm f2.0 Summicron on an M3 in the past. While I think the rangefinder approach is the main difference between Leica M and Nikon SLR, you might want to compare
this B/W shot to the other link. This lens was traded for other gear years ago, and the unfortunately broken M3 has also been sold, making this shot from college the oldest shot in my portfolio.
I wouldn't directly compare Leica M and Nikon SLR, since the requirements of making the lenses is very different. However, I think Nikon do a great job with some of their late 1970s to early 1980s lenses. Be careful of sample variation; I went through four f1.4 AIS lenses prior to finding a great example. Also, I would gladly buy a Leica M again, though probably an M7, and more likely with a 90mm, a combination that the Leica reps loaned me a few years ago; unfortunately, I don't know when/if that will happen, since I shoot mostly large format now for my clients. Lastly, I highly recommend using a lens hood on whatever lens you decide to purchase; it will make a noticeable difference in your images.
Ciao!
Gordon Moat Photography