Elements and their percentages in steel:
D2 = C 1.50%, Mn .40%, Si .40%, Cr 12.0%, Va .95%, Mo .90%
440C = C 1.00%, Mn .45%, Si .30%, Cr 17.0%, Mo .50%
IIRC chromium is the element that provides the “rust-preventive” qualities in steel (with carbon providing the hardness). I see that D2 has substantially less Chromium than typical “stainless steels” but not being much of a metallurgist I wouldn’t know just how rust-preventive 12% is compared to 17%. If you can tell us, how much Cr is in the D2-E? Is it closer to 17% or so? For what its worth, I consider D2 to be a great steel as well, since I prescribe to the “take care of your knives” theory of rust prevention.
BTW this info was from Admiral Steel. Different manufacturers will have slightly different percentages of the components in steel.