I'm currently trying out both the SL1 and M (to use in addition to the 5D3). In most situations, the AF on the M is not nearly as bad as it has been made out to be, but if there is motion involved (even slight) sometimes it just will not lock focus (or even get close). I'm really hoping the new firmware will resolve this.
The SL1 is great all around, except that to my knowledge there's no compact ~35mm equivalent lens for it. I wish Canon would release a 22mm (or similar) EF prime. The 40mm is the lens I leave on it, but for I would prefer a wider lens for general use. I haven't used the contrast based live view AF on the SL1 in order to compare it with the M.
My wife took the SL1+40mm to a concert and loved it, and got great results. Based on what I've seen from the M's AF, I think that it would've just been frustrating to use in the same situation. One other note about the SL1 - the battery life seems to be very good - at the concert she took over 1K shots (RAW, Manual, AI Servo, No flash, no Live View, 2 second review) and the meter didn't drop at all.
I'm keeping a list of the pros/cons of each as I use them, and I plan to share it once I've used them both a bit more (and once the updated firmware is out). Assuming the improved AF doesn't disappoint, I'm almost certain the M will be replacing our S100, and the SL1 will probably find a place in the bag as well.