Standard comissions come with a tailored or edited background and more impressive character effects. The background may include multiple added objects, but not multiple shots of characters. (Note the example on the right is one character shot "mirrored" to look like three). I don't have an example of a standard wallpaper yet, but, I'm sure that will soon change ! Alternatively, you can have two character shots in the same piece, but it must be with a simple background.
Advanced comissions come with multi-layered and non-uniform backgrounds and detailed effects. These comissions can take a little longer and may include multiple character shots (Max 3).
Put simply, Wallpaper pieces are bigger and Landscape, intended for your Desktop backgrounds and the like, while personal sizes are better for Profile Pictures and single-character focus. (A small character can easily be lost to a large background if it is the only character in it)
There is no pricing difference.
Project-length pieces can include anything up to an entire collage of enhanced characters, for example.