Truth be told though, the 0.5 ms response time different won't be significant, nor will the 240Hz vs 120Hz refresh rate, at least not necessarily in the case of refresh rate.
You would be better served by posting in the LED sub-forum @ AVS to see what the pros there say about the minutiae of difference between these two models. Here's a link:
CNET has a great review for the M3D550KD, there is only one problem. Despite the review only being 3 months old, the TV is no longer being sold. Instead Vizio is selling a M3D550KDE. You might assume this is just a minor change, but if you look at the specs there are some differences, such as refresh rate. Now, I realize for most manufacturers, refresh rate isn't very honest anyway; however, with that said I feel this demonstrates that the KDE is actually a completely different television then the KD and not just a newer version of the same model. The KDE also has no reviews on it, anywhere. All of these facts combined with it coming out just in time for holiday shopping is screaming bait and switch to me. Produce a great TV with a low profit margin, get great reviews, make a different model with the same branding and sell it with a higher profit margin based on the reviews the earlier TV got. Anyone know anything about this?