Well, for a 40 you will find that they bring up the 50mm framelines on an M. Both the Summicron C and Nokton are very nice. I had a Minolta Rokkor CLE 40/2, basically the same as the Summicron, different filter thread, focusing cam and multi coated, which I loved so I am biased towards the Rokkor. In the same price range you can't go wrong with a 35 Ultron or a nice condition black Canon 50/1.8 which should sell for a little less. I think the BGN price at KEH is a little high, the last couple I saw sell have gone for less than $300.
How good are the CV lenses? I would say to look through images here, flickr..... look for images from these lenses. Let you eyes pick out something that you like, or don't like about them to help you make the decision.