I have both, there's no better camera, it just depends on what you prefer.
The ZM feels a little inferior in operation, it's well built, no doubt about that, but the shutter dial clicks a little weird, the advance not as smooth, the RF patch not always in the middle and can be more prone to misalignment. I also find the exposure compensation dial on the ZM too fiddly.
But the ZM has a bigger finder, aperture priority and the 1/2000 can come in handy. It won't burn a hole in the shutter if you point it into the sun, and since you can turn it off properly, you can leave a soft release on without worrying about it firing in your bag.
If I have to sell one, I'd keep the Leica, especially since it's kind of a special gift, but that's just me.