Hi Lester,
The DMK31* has a bigger chip so you get 1024x768 resolution, but a smaller pixel size (4.65um) - what that means in practise is a bigger image, but less well depth and less potential dynamic range - therefore less sensitivity.
For planetary imaging, the DMK21* is better than the DMK31 or 41 series.
Check out my reviews for comparisons of the 31 and 41's versus the 21.
Review of the Astrovid Voyager X (has the same chip as the DMK31)
Review of the DMK41AF02 (and comparison vs the DMK21).