I used to own the DMR, but I have not shot the 5DII. I use the D3 now, occasionally with R lenses. While I like the color better on the DMR, I don't think there is a large difference in sharpness. The advantage of full frame and dramatically better ISO performance mean that the D3 was a better choice for me. The DMR was and is a great camera for shooting R lenses in the standard focal lengths at ISO 100-200. In almost all other aspects, it is hugely outclassed by later SLR's. If you are happy using the 5DII now, I don't think you will gain much in the way of real world advantages (other than not having to stop down meter).
Advantages for DMR/R9, in my mind:
1. No stop-down metering
2. 16 bit color
3. No AA filter means very sharp files out of the sensor.
4. No messing with adapters.
5. Can use the R9 with film, where it really is best.
6. Can use all the R lenses.
7. Nice ergonomics, made for manual focus, good, solid camera forms its basis.
R9/DMR disadvantages
1. Big, heavy, still expensive and ending its repairable life -- Leica is only using donor DMR's for repair at this point. At some point in the not too distant future, they will run out of certain parts. This may be fine if yours does not break, but if it does, you might be out of luck.
2. Dismal ISO performance compared to cameras like the D3, 5DII. 800 is the upper limit.
3. Outdated technology -- tiny, low rez LCD. Poor battery life. No feature set with things like live view, custom functions, and so on.
4. 1.37x crop, uses framelines in the viewfinder, so you don't have 100% exact framing (at least mine didn't). Wide angles become normal, superwides become moderate wides. Like the 19mm becomes a 26mm. That is a big difference.
5. The camera is modular, so the weather sealing is non-existant, you can have registration problems (the sensor not being in the right place), the focusing screen framelines might not be perfectly aligned to what the sensor records, it is a much larger camera than integrated cameras...it has a rather weird shape as well.
6. Slow, slow, slow. The frame rate is slow, the images take a long time to show up on the LCD, scrolling through images is slow. Looking at detail is very slow. Even changing ISO takes a few seconds!
7. Only one card slot, and it is SD -- I believe it is limited to 2 or 4gb, but they may have updated. I do not recall.
8. There will not be any more firmware updates -- what you have now is the best it will be.
9. The newest cameras are now several years old.
10. The camera is temperamental. My first one developed a sensor line problem after only a few thousand shots. It went for repair, came back with a different problem. It was replaced, then the replacement had outrageous (even for the DMR) noise at all ISO's. This was repaired and it did fine for 2 years. Then it decided to temporarily stop working while I was photographing in a helicopter -- the worst possible time. That was the final straw for me.
11. Lower resolution than the 5DII. The D3 and DMR are essentially equal in real resolution...the DMR files are sharper out of the camera, but if you do minor sharpening to the D3, you will see equal sharpness and about the same amount of true resolution.
Overall, I think the DMR was a good camera at the time, and it still does things well, but it had many things going against it, from the firmware wars (between Imacon and Leica) to the sort of cobbled together nature of the design. This camera came out in 2005, was conceived in 2003-4, and I am sorry to say that it is now showing its age. If you are happy with the 5dII, and don't already have a full R system (and you shoot film with it), then I would recommend against getting a DMR unless you can get it with the camera for less than 2000 USD.