Without knowing the settings which were used by each renderer (and thus stuff like the BSDF materials, sampling techniques, random number generator sequences, e

Without knowing the settings which were used by each renderer (and thus stuff like the BSDF materials, sampling techniques, random number generator sequences, etc, etc), it's very difficult to say. All these things play a crucial part in how efficiently a renderer can produce an image with as little noise as possible. But it's also possible to look through the noise (ignore it), and look for issues - i.e. the Classroom scene is very odd as Appleseed doesn't have soft shadows (what light type was used? PhysicalSky/Sun? HDR IBL?), and the Tungsten illumination from the environment is less warm.