I think a 24mm pancake would be a perfect walk around lens for a rebel. An SL1 with a 24mm pancake would be a mirrorless killer.
My 6D is a mirrorless killer with the 40STM! Mirrorless killer as long as we aren't including the A7 series! I'd still take my 6D over the A7/A7r, but the A7s could easily earn a spot in my camera bag for night work.
Here's a size comparison.