I don't have the 6D, but I have a 5D II, the pancake, and the Sigma 35mm f/1.4. The sigma really is a great lens, but for the price, you simply can't beat the 40mm pancake. In fact, I used the 40mm lens on a professional shoot not too long ago (paired with the 5D II) when shooting a personal training studio - and the images were great.

Pick up the 40mm lens - you simply cannot go wrong with it (IMO).