I have the 35 Lux ASPH, the Noctilux, and the 50 Summicron (current version). The 35 Lux ASPH is a stunningly good lens. From the day it arrived almost two years ago it has been the one I've used the vast majority of the time. If I could only have one M lens, there's no question but that one would be the one. It's far sharper than the Noctilux and renders in a far more conventional way - none of the painterly, surrealist kind of look that the Noct can sometimes serve up (that's not intended as a knock against the Noct, as that look has a lot to do with its signature charm). Do be aware that a lot of folks prefer the pre-ASPH Lux for creamy, glowy bokeh.
Owning both a Noctilux (which I won't get rid of) and a Summicron 50, it's pretty crazy for me to even consider the 50 Lux ASPH. But I love the 35 Lux ASPH so much and because the 50 Lux ASPH reputedly has a signature very similar to it - that I'll probably bite the bullet and do exactly that later this year.
Bottom line, I don't think you can go wrong with either the 35 Lux ASPH or the 50 Lux ASPH.
Jeff