I'd say get the 25, especially if you want the speed of the 2.8/21 over the C Biogon's 4.5. That C Biogon is the reason I'd consider a 21, actually.
I used the ZI and RD-1 on a three week trip through France, Italy and the United States and only took two lenses, the 25 and the 50 Sonnar. I thought that was perfect, really.
Another advantage of the 25 on both the ZI and the RD-1 is that you can get away without using an external VF much easier than you could with a 21. But I still have CV 25 and 35 VFs to go with the two lens/two body combo I mentioned.
The 25 on the RD-1 is closer, I feel, to 35 than to 40. Plus, the funny thing about cropping is that although the FOV is 1.5x, the perspective still remains the same as the original wide angle. For that reason too, 25 is better I think than 21, since the former is probably the last wide angle with a slightly normal perspective.
I have an 18 for going ridiculously wide. It looks closer to a 25 FOV on the RD-1 than a 28.