I have a few summiluxes and really love the E43 version. They are very well made, about the size of the 50 summicron, and have very nice optics. I've never shot the 50 1.1 Nokton, but the E43 Summilux has never disappointed, even wide open, where people say there is potential focus issues. I personally have not noticed focus shift, but to be honest, I shoot very much on the fly and never ever shoot on a tripod. With that said, if you have the money to pick up a 50 Lux E43, I would highly recommend it. I now have 2 of them because they are so wonderful. I also have the E46 version, but it rarely gets used. No particular reason for this, but I just prefer the E43 without the hood.
The last thing to possibly consider, other than optics, feel, etc. is the potential "investment" you make into the E43 vs. the Nokton. The E43 or E46 will hold its value far longer, and may even appreciate in the future. This might be something to consider if you choose to go this route. My thought is, go for the Lux, and if you decide you want something faster/more modern in the future, you can always sell it for what you paid, and possibly more if the market allows.
Hope this helps, and good luck on your decision.
P.S. I have shots with the 50 Lux E43 as well as the E46 and ASPH on my Flickr page using an M9 and film M's for some (probably not the ASPH), just in case you want to compare some shots. They are not test shots (not the same subject or anything), but it might help your decision.