Frank, the 25mm Biogon ZM is one of the highest resolution lenses ever produced commercial for 35mm photography, so it probably is better until f/8 when diffraction begins to limit both lenses' resolution. But I've never used the Skopar 25mm, so I'm saying this based on Zeiss' MTF measurements and my own use of the Biogon 25mm compared to Nikkor 24mm/2 AIs and 24mm/2.8 AF lenses. While those lenses are very good, the 25mm Biogon is just stunningly better. Unlike the current Skopar 25mm, the Zeiss 25mm is RF coupled (to 0.7M, but like the Skopar, decoupled scale focus is possible down to 0.5M.)
With the 25mm ZM lens at f/5.6 details are crisp over a huge depth of field, all the way to the very corners, for landscape/cityscape photos near infinity. Tonality is very smooth with fine grained films, and color rendition is gorgeous, with very realistic saturation. It's very flare resistant. I had not realized it was capable of such subtle background blur until I saw Tripplefinger's wonderful portrait. I definitely want to explore this aspect of the lens now. It's like a whole other world I never thought to explore. Sometimes I get trapped trying to keep everything in depth of field in a scene, ignoring when it might be interesting to have more blurred areas that can make the subject pop!
The size of the Biogon 25mm lens is going to be a huge difference to the Skopar, with the Biogon being much larger in length diameter and weight. It's long for an RF lens, but not too huge, projecting 47mm forward from the lens flange/camera body and taking 46mm filters (narrower than the Leica 24/2.8 ASPH with its 55mm filter size).