Johann
The real problem, is that you should keep the Nokton AND buy the Biogon...
There are 3 good reasons for that:
1- the Nokton is a very good lens, and in fact it is the fastest lens around, as you can shoot it at 1/15th easily, what's more the dof wide open is still useable for a reasonably close portrait and the bokeh is very pleasant
2- the Nokton is the most universal lens available, as it can be used across a very wide range of apertures and is performing well close up and at infinity - if you have to take just one lens with you, then that's the one - just get a 400 iso film, a ND or a dark red filter for outdoors, and you are set
3- if you can take 2 lenses with you, then the 28 and 50 mm combo is outstanding, so a good 28mm will expand your photographic possibilities especially outdoors, while a 50mm will separate better human subjects. If you feel the 28 and 35 mm are too close, just go for the Biogon 25mm and you will have a more versatile kit, particularly for landscape.
A Nokton 35 shot at f1.4:
http://www.flickr.com/photos/59177039@N00/2265887144/sizes/l/