As of right now Keymod has a much larger market for second hand parts. It's been out for a few years now, so mounts and accessories are plentiful and easy to come by. With this being said, I believe M-LOK to be the superior design. It's simple, easier to manufacture, and easier to mount accessories to. Not to mention it looks much cleaner and not like industial shelving. But... due to the fact that so many manufactures have spent time and money to begin producing Kemod rails and accessories over the past few years, i believe it will be a while before M-LOK truely catches on and takes over the market. So, my recomendation is, if you want something fast and cost effective go with the Keymod. It will get the job done just fine and theres nothing wrong with it. But if you dont mind spending a little extra and waiting on aftermarket accessories then go with the M-LOK. it would be the better long term investment. Either way you cant go wrong, its just personal preference.