For what is worth, I got a M10R to add my kit as a backup for my Phase One IQ4 + XT setup, when I moved to Phase One; since I got it before the move to P1, I had a little time when I used both the M10R and my now departed X1D together.
You surely know that already, but I believe it's worth to mention that the systems are totally different in use, and in output as well. In short, just to mention a few key differences (for me, at least), the X1D is a modern camera, albeit slow, designed from the ground up with autofocus, EFV offering precise composition, comes with zoom lenses, ultra-wide lenses and tele lenses, offers RGB histogram, better implementation of long exposures, and more; the Leica M10R is a modern version of a classic design, is manual focus, comes with a rangefinder which doesn't let you compose as precisely as the X1D (you need to use the back screen & live view for that, or attach a Visoflex), doesn't offer zoom lenses (short of the Tri-Elmars) and makes it very difficult to use lenses wider than 28/24mm and longer than 75mm for framing and focussing (short of using the back LCD or Visoflex, in live view). The M10 system is smaller and lighter than an equivalent X1D system covering the same field of view; the X1D allows you to charge batteries in camera via USB-c, makes it easier to replace batteries and SD cards; and so on.
So, depending on your intended use, one system might suit you better than the other. For me, as a backup for my X1D II I used a second X1D II, in fact I used two cameras at all times in the field, to minimise lens changes; as a emergency backup to my Phase One, I found that the M10 R works pretty well, albeit with not comparable image quality. The workflow is similar, with both relying on manual focus and use of the back LCD; a M10R kit replicating my 23mm, 32mm 50mm and 90mm Rodenstock lenses with all-Voigtlander 15mm, 21mm, 28mm and 50mm is very small and light; plus, it allows me to have a hand-holdable camera for travel snaps, backstage photography, and so on.
Coming to image quality, I second what others said: the X1D's files are better in terms of dynamic range, color (subjective, I know), and cleanness if you use long exposure; and, again subjective, I think they have a more "neutral" look than Leica files, which I like as a starting point for my post-processing work (but others won't, I am sure). That said, the M10R files are beautiful, and - again subjective - for me personally it's always a joy to use rangefinder cameras, and a rangefinder kit with fast lenses offers something in terms of look that a X1D kit (or a Phase One kit) couldn't do.
Perhaps, in full agreement with Dante, I am trying to recommend you get both
Hope this helps, best regards
Vieri