As someone who owns the M10, I don't like its stereo separation either -- it's good as long as your sound source is near the recorder. The more that moves away, the more mono-like your recording becomes.
I actually had the opposite experience. Listening closely to an M10 ambient recording I have posted in this thread, I felt that the stereo spread seemed wider when there were no sounds nearby.
Really interesting -- my own time spent with the M10 proves the opposite. The more distant the sound source, the more mono-like M10's recordings become. This is not necessarily bad or unfavorable -- I sometimes prefer that feature of the M10, but it's never been my number-one choice when it comes to stereo recording via built-in mics.
Well, we all hear differently.
I think stereo recording via built-in mics is always a compromise no matter what the configuration. Cardiods in x-y on a recorder are usually not far enough apart and give only slightly better stereo spread if you're recording a band. And they are poor for nature recording because they reject so much of the ambient sound. I've had the same problem even using an external stereo microphone like the BP4025 for ambience.
The M10's internal mics may be its Achilles heel for some, but all-in-all, for the OP, I'd say it is the best choice of recorder for nature sounds because it has such inherent low noise and the omni mics capture so much of the environment. The M10 is a great sound-catcher, or sound-collector. You can keep it in a bag like a point-and-shoot camera. Long battery life, hardy, and teams up well with preamps.
And there was one in the yard sale -- last time I looked -- for only $150!