The Digital Upgrade is a way of buying nothing but Greymoor - for current players who already have most/all the current chapters and don't want to pay for them again. For anyone else it's almost certainly better value to get the Standard Edition (which I think can be added to an existing account - for example if you have the base game but nothing else and want all the chapters).
I remember Guild Wars 2 made the mistake of bundling the base game with their first expansion and upset a lot of players by not offering a separate version. Even after it was explained that's because the base game had recently been made free-to-play (although free accounts have lots of restrictions that paid base game accounts don't) so the expansion would cost exactly the same either way a lot of people were still unhappy about it and complained they were being charged twice for the same thing.
So even though there's not much of a price difference it's probably much better for ZOS to offer both.
PC EU player | She/her/hers | PAWS (Positively Against Wrip-off Stuff) - Say No to Crown Crates!
"Remember in this game we call life that no one said it's fair"
And if you are missing the previous chapters an want em, just buy the version with base game and old chapters for your existing account. You will not get duplicates but do unlock whatever bundled you did not already have.