Hi Henrik. To make things even more complicated, there are two completely different versions of the Super Takumar 35mm/2.0. The first version is a big heavy lens with a 67mm filter ring, and the second version is much smaller with a 49mm filter ring. The one I own is the 67mm version, so that's the one I'll be talking about here.
The short answer is easy: if you can get hold of the 35mm/2.0, just grab it and never let it go. It's a truly great lens.
The problem is that the 35mm/3.5 is a great lens too. At first glance, people often prefer the results from the 3.5. It has a slightly more saturated and contrasty colour rendering than the 2.0, and it has quite strong edge contrast that gives it a feeling of super-sharpness. If you want to blow people away with their first look, the 35mm/3.5 is a lens that can do that.
The 2.0 is a much more subtle lens. The colour rendering is more natural and there's less of an immediate "Wow!" factor. The 2.0 doesn't over-emphasise the edge contrast, so at first glance it can seem less sharp than the 3.5. It's only when you compare the two lenses at 100% and stopped down a bit that you realise that the 2.0 actually resolves more fine detail than the 3.5. The 2.0 aims for fine micro-contrast and a sense of three-dimensional realism, while the 3.5 aims for high edge contrast and a sense of extreme sharpness.
Pixel peepers would not want to use the 2.0 wide open. Personally I'm not a pixel peeper, so I use it wide open quite happily.
If you want to see some excellent examples with all the different versions of both lenses, have a look through the Takumar Club here on Pentax Forums:
https://www.pentaxforums.com/forums/122-lens-clubs/31601-takumar-club-886.html