My point about volume of sales is purely anecdotal, I simply do not have access to the info, working off of what I have been told and what seems to be supported by market presence.
Keymod
Pros:
Open source, not restrained by a TDP, which allows a manufacturer to improve upon the mounting method.
Shows better alignment of mounted items, as the attachment nut is conical and tightens into a beveled recess.
Cons:
Open source, not restrained by a TDP, which allows a manufacturer to screw-up the mounting method.
Requires higher than usual toque for a small screw to avoid loosening, but must remain low enough to prevent damage to the nut or handguard.
The attachment nut and mounting slot must be able to withstand significant torque without deforming, which makes both highly sensitive to material and hardening processes.
To reverse an item, the nut recess must first allow it, and the user must loosen and flip the nut.
M-Lok
Pros:
Patented and controlled by Magpul, which ensures that everyone that is making an M-lok accessory that bears the M-Lok logo has to have been approved by them, which keeps everyone at the standard that they want.
Attachment method can bear very high torque, ensuring that accessories won't loosen if mounted correctly.
Attachment hardware can withstand high pull force.
Items are immediately able to be reversed when mounting.
Cons:
Patented and controlled by Magpul, which prevents outside manufacturers from changing the attachment method for a specific purpose.
Attachment hardware can be shifted (without loosening) with lateral leverage.