I own both the Sigma 35A and Voigtlander 40mm Ultron pancake.
For "street" photography I find the Voigtlander to be far and away better as it is way more discrete and a joy to MF. For most other things the Art is a slightly better lenses (truly, the Voigtlander is stellar as well) with the largest difference being their size.
I picked up the Voigtlander on Ebay for $200, they can be found quite cheap a lot.
Did you buy the Voigtlander in EF mount or M42? I always thought about trying out that lens for the same reason as it's smaller than the Sigma 35A and $200 seems cheaper than what I remember of what that lens went for.