Finally, what’s the difference between either and the Embraer 170…what design and engineering differences make one a “RJ”?

From my own point of view, it would seem to be the airline’s contracts with their pilots!

In terms of efficiency though, the A318 and the 737-600 have shown that a fuselage that fits 3+3 just doesn’t scale well down below ~140 seats (single class). Too much tail volume, fuselage weight and wetted area for what your getting back; although the oversized wings aren’t helping.

It’ll be interesting to see how the 3+2 arrangement goes in the CSeries,c we’ll see if that niche between 100-150 seats large enough for a full dedicated program.