You might start with Sean Reid's reviews. Probably worth the money if you're considering these lenses.
My own views, far far less informed than Sean's:
the Nokton is a tremendous value. The Biogon is (at least vs. a Leica lens) a very good value.
If you photograph mainly in color, and don't really need the extra stop, get the Biogon. The ZM lenses are just wonderful in color. In B+W? You *might* be happier with the Nokton.
If you regularly shoot highly detailed subjects (architecture, landscapes) on 100 speed film, perhaps with a tripod, you probably want the Biogon. In addition to its low distortion it has low coma & astigmatism, and very little curvature of field. The Biogon is really at its optimum from f/2.8 to 5.6, and is a bit weaker at f/2. At f/4 the Biogon has similar MTFs to the current 50mm sumicron or the 35 summicron ASPH, but with less curvature of field than either one. At f/4 and f/5.6 the Biogon's overall performance is eqalled by few other 35's, and it's probably surpassed by none of them.
The MTFs and other data for the Biogon are
here. Ken Rockwell has a reasonably good nontechnical article on how to interpret MTFs
here. The short version is that it is hard to criticize the Biogon on this basis.
If you tend to shoot handheld at ISO 400 or higher (especially B+W, especially Tri-X or HP5+), and you find yourself often using shutter speeds below 1/125, I can't think of a reason not to get the Nokton. At wide apertures, the difference in optical quality will have much less effect on IQ than a low shutter speed, and an extra stop is an extra stop.
If you to shoot in adequately bright but difficult light (say, sun in the frame, or lots of specular reflections e.g., bodies of water, or cars glinting in the sun), the Biogon will probably be better at suppressing flare.
You will probably also want to look closely at the rendering of both lenses, there are examples here and all over the web. In terms of bokeh at wide apertures, I don't think one lens has it over the other. They both have adequate, but not superlative bokeh (IMO). In the large majority of shots the backgrounds will be very nice with either lens, but both can be forced to misbehave. It's pretty easy to find examples of this, and once you have experience, you'll know what situations cause problems. They do render differently. You'll just have to look at examples to see what you prefer.
Look at your own best photos. Do you see usage patterns that might help you to steer your choices? For example: for many years I was on the fast lens bandwagon, but a big scanning project demanded a reevaluation: most of my best images, especially with the 35 and 50 mm lenses, were taken at middle apertures -- not wide open. FWIW, I currrently use a Biogon-C 35/2.8, which I love because of its rendering at f/2.8-5.6. (I'm thinking of getting the Nokton 35/1.2 or maybe the 40/1.4 for low light, but it's a lower priority. Since you asked about f/2, though, I cannot recommend the Biogon-C
😛 )
One last thing. I've assumed you're shooting film. With a digital sensor, I'd personally skew towards the Biogon. With film, the Nokton might more likely be my choice.
