@Andy_T
just for the fun of it, ask the same question in the kindle forum.
You would most probably get different recommendations.
Finally, it is you who to decide which features are important for you.
Here're couple of my thoughts, though (to make it more complicated
).
1) Wifi is a plus but do not expect a computer experience.
2) More RAM and processor should theoretically result in a better reading experience but interplay between software and hardware seems to be very important. I remember seeing a thread that with some pdf files KDXG still has beaten the M92.
3) Availability of an keyboard can actually be a plus.
3) M92 still gets updates, KDXG has seen practically no updates from the very beginning. Some M92 users consider the M92 software half-baked. I would not recommend the KDXG out-of-the-box to anyone. What makes it shine is a alternative software kpdfviewer. It has many features missing in both kindle firmware and (judging by the M92 wish list) M92 firmware.