I'm 173cm and have a 75cm bb to saddle top, very similar to Howard. I'm a bit more legs than torso. Riding medium arkose with 80mm stem and the fit is great. With 25mm setback Seatpost and stem slammed (well, on to the ~16mm conical headset spacer) I've matched the fit on my size S canyon ultimate (on which is a 110mm stem), albeit with 10mm extra stack, which I think is perfect for the type of bike this is!
I had the same questions as you when buying, and I'm pretty sure either size could work. Not sure if toe overlap is a risk with running the size s.. Worth checking. I ended up with the M simply because that was what was available on ebay at the time of buying. Hope that helps.