The 40mm Summicron-C was made by Leitz in Wetzlar, probably with some parts from Minolta, but I've never seen a breakdown on that. The 40mm f:2 M-Rokkor was made in Japan by Minolta.
Originally Posted by Anscojohn
To answer the original question about a 40 Summicron vs a 50 Summicron, you'd have to take into account different generations of 50 Summicrons. The 40 Summicron-C was considered a bit of a 'sleeper' by many when it appeared. I've been very happy with the performance of mine for the last 25 years. You can't really go wrong with any good condition Summicron of any focal length. Others with more experience across the different models will be more qualified than I to make those comparisons.
Here's a place to find one opinion based on wide experience: http://www.cameraquest.com/mlenses.htm