I've owned and used both the Biogon 35mm f/2 and the Voigtlander Nokton 35 1.2 (Version II) extensively on my Bessa R3a, M6, and M8.
I shoot a lot of dusk/nighttime stuff so I've always been a speed junkie. The Biogon was great but it produced too "modern" an image for my taste, and it was also just a little too contrasty for my taste on film. Underexposed, the Biogon tended to crush shadow areas into blackness more readily than the Nokton. Dare I say, the output from the Biogon + M8 just looked too much like the output from any number of "modern" lens and sensor combos made by Canon. I gravitated towards the 35 nokton 1.4 MC for its astigmatism aka "swirl" and classic look...I wanted my images to have a bit of signature to them, if I wanted "boring" and modern I'd just use a 5D+35L. 😛 The Biogon (and eventually the 35mm 1.2 nokton) were my go-to "modern" looking lenses. I eventually settled on the Nokton 1.2 (version II) because wide-open, it wasn't as "perfect" as the Biogon, although it is still a hard lens to find serious fault with. The 35mm nokton 1.2 was a great compromise between signature and IQ, without having to spend $3k+ on a Summilux ASPH...hell I basically spent $3k on the nokton AND my M8.
The 35mm 1.2 was a LOT sharper than I'd expected and as far as I can see, it isn't as contrasty as the Biogon up to about f4. From around f5.6-on I have trouble telling the two apart sharpness-wise, although the Biogon always looked slightly more contrasty (hence slightly "sharper"), and was more likely to blow highlights the way I shot it, both on 35mm and digital. The merits/caveats of the 35 1.2 and the Biogon have been demonstrated all over RFF, so I won't talk about it too much here.
Personally, I preferred the Nokton 1.2's background-destroying shallow DOF and its milder contrast. Both lenses were sharp enough for my needs, and then some. f/1.2 isn't something you need everyday but when you need it, it's always great to have. The Biogon was contrastier, slightly sharper at wide apertures, and under the right conditions exhibited that Zeiss 3D "pop" that some like. I kept the Nokton for its speed and versatility.
Both lenses are excellent choices, so there's really no right or wrong answer here. The decision would probably come down to your own taste.
In my opinion, you'd be better off with the 35mm nokton If I recall correctly, the M5's metering was designed to work with the 50mm Noctilux under low-light conditions. If so, why not pair it with a fast 35mm?
One big difference between the two ergonimically (besides the obvious things like size or weight) is that the Nokton's focus ring has a much longer throw and lacks a focusing tab (like the Noctilux). Weird unrelated side note, I got to demo a brand spanking new Noctilux and the focus wasn't as smooth as the Nokton's (!!!!). The Biogon has that weird metal nipple on its focus ring and a "traditional" short rangefinder focus throw which should feel familiar to M shooters, while the Nokton's throw is more like a Nikon AI lens. Both the Biogon and the Nokton's focus feels nice and smooth in actual use. Oh and the Nokton close focuses to .5m, where most (if not all, I'm no Leica expert) Leica RF patches only focus to .7 meters. This feels weird when you're close focusing for the first time and you feel the ring keep moving even after the patch stops, and the.5m focus stop still inspires a little "did I get it?" nervousness whenever I take a close shot. The Nokton's longer throw takes a little getting used to, and makes the Nokton a little slower if you're into the whole zone focusing and "from the hip" thing. After readjusting to the Nokton's longer throw, I found that it really helped me to get more accurate critical focus, which is important when shooting with DOF that narrow.
One last thing to consider: I know you said you'd use it with your M5, but if you ever plan on using it on a digital M, try to find a copy of the 35mm Nokton Version II, as the first version has had issues focusing on digital M bodies. The Biogon worked fine on every camera I mounted it to.
I don't know if you're dead set on the 35mm focal length, but I've also had a lot of experience with the CV 40mm 1.4 Nokton SC (came with my Bessa)...it doesn't vignette as heavily or show the kind of astigmatism that the 35 Nokton Classic does, and in the copy I've tried the 40mm seemed to be a hair sharper at wide apertures. Might be worth considering as well, since it's even more affordable, and I love the ergonomics of small RF lenses.
Hope this helps!
