GXR-M if you want to use the M-lenses and like a bit of tele-action. It is also more quiet, smaller and more discreet. The image quality compares very favourably with the M9 and it retains the unique signature look of each lens. It's a good backup and second camera to a M9, and makes a super camera in its own right.
X100 if you want a rangefinder style body, excellent image quality and autofocus. I love mine.
But if you want something that complements the M9 rather than performing similar duties, I've become very fond of the EM-5. It does everything that the M9 can't, like good high ISO, decent quality video, autofocus, focus closer than 70cm, costs much less money, and is weatherproofed. Get a lens like the Pana 12-35 and you have 24-70mm focal length range in a weatherproofed package.
There are even m43-M adapters which will allow you to mount M lenses on the EM-5. When I get the 12-35, the EM-5 will be one half of my walkaround setup, and the M9, X100 or GXR will be the other, depending on what I want at the time.