The 40mm 'cron is to me a nearly perfect match with the 35mm frames in my M6 at distances over a couple of meters. I'm very happy with mine except for a slightly sticky focus ring.

I've seen a number of people compare the 40mm to the 35mm Summicron IV ('King of bokeh') in terms of sharpness and bokeh but I haven't compared myself as I don't have the 35mm. Biggest drawback of the 40mm 'cron is the filter size, the now non-existing Series 5.5 (but you can use 39mm most of the time).

The 40mm is exceptionally good value and your decision should be based on whether you want 35/40mm or 50mm FOV, not which lens is better. They are both very good.

Here is more information re 40mm Summicrons:
http://photo.net/bboard/q-and-a-fetch-msg?msg_id=00KDUs
http://photo.net/bboard/q-and-a-fetch-msg?msg_id=004X9y