I have the 18, not the 21, hence I cannot speak with assurance about the two. But I have the impression that the 2,18/21 will vignette less than the 18 on the RD-1. Consider whether that matters to you.
I find the 18 works well with a 25mm CV finder. But I prefer using the RD-1 without external VFs (hence I generally never go lower than 25) and there is something about the crop factor that makes it harder for me to like super wides on the RD-1. Although the crop factor turns an 18 into a 27, the perspective is still that of an 18. Others seem to like super wides very much. Perhaps the problem is that I just haven't spent enough time with it to learn how to use it (but first resulst didn't really entice me, either). YMMV.
If I were going for a new lens dedicated to the RD-1 and not a film camera, I'd definitely choose the 2,8/21 over the 18 based on what I know. For film cameras, however, it is whole different story. (I've also seen fantastic pics with that lens from the M8). The 18 is fantastic. And as for 21s, I personally hope to acquire one day the 4,5/21 for use with film...
