I had both setup in a single space once. The Martin Logans were used as part of a surround sound setup, while the KEFs were part of a dance floor setup.
I would say that's pretty accurate for how I would use them again. The KEFs can get loud, but aren't quite as clean as the MLs. Neither is intensely loud, so if you want a night club, then get something completely different. If you want a bit more of a full range speaker that's a bit brighter, and more responsive, then the MLs really fit the bill, and IMO, are cleaner for more normal listening surround sound. But, when the audio goes louder, they tend to be a bit brighter than I like. That's when the KEFs really shine. When the volume is up louder, they tend not to be as bright, and are more enjoyable for louder listening.
That's my take at least. I've used the Q900s a few times and I like them. I think they also do a bit better in a larger space, but they don't sound as full at lower volumes. That's where the MLs shine.