Hi and
to the forum.
More information regarding the dropout width can be found
here.
I'm pretty sure that the front mounted 901 is not suitable for disc brakes, contrary to what it states on the above link.
The larger MPIII produces more torque, which will provide more assistance on hills than the 901, but the downside is that it causes more resistance to pedalling when not using motor assistance and may also have a slightly slower top speed.
Alan