I purchased the M10. But when I was comparing the differences between the M10 and the D50, I was looking at them as solid-state 2-track recorders, not as microphones. In my mind the M10 was the winner. I saved $200 that went towards more gear in my rig that was better designed and more practical for the situations I was hoping to capture.
The M10's a good pocket recorder that's convenient to have with you every day in case you hear something awesome in your travels that you have to record right then. However, you are looking to specifically go out and record "ambiences, birds, waterfalls, animals..." in stereo, and I wouldn't trust either the M10 or the D50 to handle those tasks on their own.
For those kinds of planned recording trips I went out and purchased a SD MixPre. This provides phantom power and XLR inputs allowing you to use any microphones you desire / can afford in a proper stereo configuration. The difference is substantial. Connection between the two is via the MixPre's 1/8" Tape send /rtn and the M10's line in / headphone out.
Now I have the best of both worlds, a convenient pocket recorder, and a well-engineered field mic-pre that allows me to use professional microphones in a proper setup.