I have both. I don't have a preference. I think Keymod probably has better availability of specialty mounting trinkets, but I think I read the USMC went with M-Lok, so expect that gap to disappear. I do remember having to hunt for aluminum rail sections, for M-Lok, because Magpul makes most of their stuff plastic...Tough plastic, but plastic. The determining factor for me was the tubes themselves. Those Matrix tubes, at JoeBobs, are hard to beat at that price! The black finish is a bit chalky, but I just painted one FDE, to match FDE furniture, and in that application chalky black didn't matter. I have them on most of my rifles now. I started out with a couple of MIs, one in Keymod, and one in M-Lok, but at 2.5x the price of the Matrix tubes, I stopped buying MIs, and outfitted the rest of the rifles with Matrix tubes. If you go with a Matrix tube, then Keymod is your only option.
Squeeze