Looks a lot more interesting to me than the under performing M series.
Just made this gif, the b is much smaller than the T4i.
A good comparison between both cameras that you provided here.
EOS b opens the opportunity to use 1 or two secondary cameras in
a standard photo backpack - by putting them with a lens attached
into a standard lens compartment. This might be the "intelligent rear
cap" for EF(-S) lenses and makes a really cute combo with the 2.8/40mm!
If I had not invested into a EOS 600D(T3i) it would be interesting ... if
it has the video zoom mode (3x) with a 100% crop of the sensor center
region (5.6 400mm with 2x TC results in 3840mm focal length!).