User preference for sure. There's some reviews about keymod breaking under intense torque but for the "weekend warrior" it comes down to looks. Aftermarket attach/parts are plenty for both. I have both and it came down to other needs for a rail not keymod vs mlok.
It is personal preference for sure, but there are some points to know.
Keymod:
The Good: Mounting accessories require little to no special attention thanks to KeyMod’s self-aligning design.
The Bad: KeyMod doesn’t have nearly the industry support as M-LOK, which is backed by industry titan Magpul, who exclusively makes M-LOK-compatible systems.
M-LOK:
The Good: You can mount your attachment to the front or rear end of the slot, as opposed to only mounting on the rear, narrow part of the KeyMod. Also, most Magpul accessories can mount directly to the M-LOK system without needing a rail adaptor.
The Bad: You have to be more careful making sure your attachments are properly aligned when mounting them to your M-LOK device.
US SOCOM tested both systems and released a report on their performance, and the news wasn’t so great for KeyMod.
For most of the duration of the test, KeyMod and M-LOK both performed well, but when it came to stress testing the two systems, M-LOK rifles could be dropped on the ground and 100% of the time, the attachments would stay securely fastened to the gun. For KeyMod, however, the success rate was a measly 33%.
The Verdict
The winner of this contest is clearly M-LOK. In fact, the M-LOK system has approximately 3x the industry support as KeyMod. But, while that SOCOM report didn’t do KeyMod any favors, the company hasn’t backed out just yet. After all, most of us aren’t putting our rifles through SOCOM levels of stress.
All in all, both systems do well as the report said. For some, it comes down to aesthetics. For others, it comes down to function and adaptability, not to mention availability of accessories for the particular rail system.
I do like the M-LOK, but I have nothing against the Keymod system.
All things being equal, go with mlok. It's the crowd favorite and magpul makes a solid product.
With that said, keymod works fine. The crane test was about as scientific as the dust test that seemed to indicate that you'd be lucky to shoot a whole mag through your m4 before it jammed.
Mlok is robust and easy to make but is arguably a heavier system. The lugs usually go farther into the hand guard so tucking a suppressor may not work. The initial setup can be a pain but once the lugs are set, it's an easy on off. The learning curve for the lug setup is pretty steep so it's not a show stopper by any means.
Some of the issues with keymod were taken care of early on with better attachments and recoil lugs built into the accessories. A good discount on a hand guard in key mod is a heck of a better deal than full price with Mlok. Some of the failures with the Crane test can probably be attributed to the relatively light weight and less material on the Mlok. You could strengthen them up with a thicker hand guard but then you're losing the weight advantage. There's no free lunch.
I've got both. I love my bcm kmr in keymod and the bcm accessories are slick. Also mentioned previously, for a sling/ bipod/ hand stop/ grip/ light for non duty use, or doesn't matter.
MLok also tends to be far less expensive...
I went thru the same issue last year, ended up that the equivalent gear in MLok was over $150 less expensive than Keymod gear. That did it for me
Noveske was one of the first Keymod manufacturers, so however you feel about them and BCM, may lead one to pay a higher premium for their work...But it looks like the VHS/Betamax fight on this one is winding down, and MLok is quite a bit ahead.