I actually tried HD and ended up getting a refund for it on Steam, then buying DE instead. The graphical settings of HD just don’t work with newer computers, the units and buildings appear very tiny. You can fiddle with some settings to force it into a lower resolution, but then everything looks very grainy and blurry. DE lets you zoom in and out, which is very convenient.
If you just want to replay the classic campaigns, DE lets you do that; it makes some minor changes but you probably won’t notice them. It has different music and slightly different sound effects, but honestly I think they are at least as good as the original. In terms of gameplay, DE is better. The AI is also vastly superior to the old one. Even the price is pretty low if you don’t get the newest DLCs (but you still get everything up through 2019).