Hi Jubb Jubb,
Though decision between M6 and M8. I have them both and would find it hard to choose between them if I could just choose one.
I definitely shoot differently with them. Using either I have to be more deliberate than shooting a dSLR. That is thinking about distance, light, aperture and shutter speed. I try not to use the aperture priority setting on the M8 too much, but it is handy for getting a quick shot I might have otherwise missed. Also with film I tend to make sure get it right the first time and tend to experiment less. With the M8 will try things or take shots I wouldn't otherwise, which is a good thing.
Purely from personal preference the M6 is the nicest Leica camera I have ever used (other two being M8 and M3). I like its weight, shutter sound and frame lines the best, but that's just me. Also if the battery goes you can carry on shooting, as it only powers the meter.
As a digital camera for an analog junky, the M8 is the closest I have experienced. Using it feels not much different to using the M6. I tend not to use the lcd screen much either, maybe just to check the histogram for blown hightlights.
If you were replacing your D200 totally, M8 maybe makes more sense. However I am sure you will love either camera.
One thing to bear in mind with the M8 - 1.33 crop factor, so if you have lenses already consider how that will effect their usage. Also budget for IR filter to avoid colour abnormalities. I ended up getting a second hand Zeiss ZM 28mm 2.8. I find it works really well on the M8 as a general purpose lens, plus on the M6 it is nice to have something wide. I use my 50mm lens on the M8 a lot too, works great for portraits and picking out details from a scene.
Compared to a dslr the M8 is a joy to carry around. I have it always ready to go with me what ever I am doing. It is my first choice grab camera now (taking over from the M6).
For taking pictures of wild life etc. though it is hard to beat a DX dslr with a telephoto prime or for macro photography. So consider what you like taking pictures of and what camera/lenses will suit your needs.
Best of luck what ever you decide on!