You have asked a very pertinent question, and one I have asked myself many times. (and also spend a lot of money on trying to figure out)
I know you stated interchangeable lenses, but what I have found for me is that once I have a another system with interchangeable lenses, is that I am temped, and often succumb, to buying many of the extra lenses for that second system. In the end, I end up building up two (or three) independent systems with overlapping focal lengths.
I have ended up with a M system, Canon EOS full frame, and Olympus EM1/5II set up. Plus specific fixed lens cameras. Fun, but a lot of overlap. (and $$$)
What I found is the X-Vario is a great daytime camera for travel, vacation, party (with a flash), and the M is the go for high resolution, night, and super high quality. (In addition the X-Vario at 28mm and F3.5 with ISO 1600 at night, or indoors, is fabulous.)
So in my humble opinion, the M system, complimented with the X-Vario is a great travel, vacation system, the Xvario is very light, (surprising light) and with 28-70, a very versatile compliment to the M System. Also, if you scale up a RAW file at 70mm to 90mm (Photoshop enlarge 1.3 times approx.) it is hardly distinguishable from a straight file but now at 90mm, not just 70mm. (seriously I mean it)
So you end up with a great combo in my opinion, which also saves you the problem of 'gear creep'. (buying more because it's there)
Lastly two comments on the Leica X system: I also have the X 35mm F1.7 but end up using the X-vario more, and 2: for the X-Vario, I set the exposure to 250 second, F stop to A, and ISO to Auto 1600 max, and everything seems to come together. Sharp (250/second) picture, Any F stop is great on that camera, and up to 1600 ISO in daylight is never an issue, ever.
Seriously, I have already spent the money already, so trying to save you some $$$.
All the best with your decision. Good luck, it is a great big candy store out there....
Dave