After countless videos and test reports, I then decided to go for the Canon 40mm f / 2.8 STM. The autofocus is quiet a lot and the speed is about the same with the 50mm f / 1.8 II. The aperture of f / 2.8 is not a disadvantage, because with the 50mm f / 1.8 II the correct focus was only from f / 2.8 because , With the difference that / 2.8 is knackscharf the 40mm pancake already beginning Aperture f. The Bokeh is very harmonious, colors and contrasts are beautifully reproduced. The build quality is better. The picture is not much bigger and thus hardly any difference to the 50mm f / 1.8 II.
I use the lens on a Canon EOS 70D with a crop factor of 1.6 (converted on full frame result from the fact 64mm). Thus portrait shots are also quite possible. In addition, there is the small size and if you want to take time only one lens, for example, for street photography, this is certainly the better choice.
All in all I'm very happy with the Canon 40mm pancake lens!