Xenon is very soft, gives that "Leica Glow" but is mainly good if you want to take pictures that look old..........soft pastelly looking colors, nice for photographing women etc. More of a COLLECTORS ITEM, then a normal daily useful lens.
I have no experience with the CV 40/1.4 but with the crop/sensor ratio factor, blah blah the 40mm works like a regular lens almost, maybe that's good? 😉
I use nothing but 50mm's with/on my M8, my favorite's are the f1.2/50 Canon LTM and the WW2 era CZJ Sonnar f1.5/50 LTM both of which are harder now to find, at least the Sonnar is, the Canon is not rare, just very hard to find in minty glass condition.
The Nikkor "Tokyo" SC f1.4/50 LTM is also a good shooting lens, along with it's slower Brother the Nikkor HC f2/50 LTM both are great shooting lenses on the M8, sharp and great colors!
Tom