Dogmusic raises very good points. Given that I have both the PCM-D50 and the PCM-M10, I can truly say that the differences are barely audible. The D50 does run two ADC's in which one runs 20dB lower than the other so that when the limiter is in use, it will perform an intelligent crossfade in the event the input is exceeded. Very clever stuff. It is indeed built sturdier so if you're accident prone or use it in potentially harsh areas, the PCM-D50 will definitely hold up to a little more abuse. The mics of both units are fantastic for internals though the PCM-D50 are just that little bit better. More importantly, you can adjust your X-Y angles with them as well whereas they're in a fixed position on the PCM-M10.
On the other hand, as was stated before, you'll get a much longer run time off your batteries in the PCM-M10 and on half the quantity. That is a major bonus when you're out in the field. Other than that, I haven't bothered to look but back in 2008, there was nothing bigger than my 8GB Sandisk MS ProHG Duo card for my PCM-D50 which bought me plenty of recording time at 48kHz/24bit. If you need more recording time than that or want to record long sessions at the highest resolutions (96/24), then you may want to consider the PCM-M10 for the fact that you can use a microSD card and it can also cross-record from the external storage to the internal 4GB storage or vice versa. Could save your butt depending on your application.
Those would be the features I would be basing my decision on. Everything else is much of a muchness. If anything, I use my PCM-M10 more so because of the reasons above and its size but the PCM-D50 will always be well looked after as my backup unit.