Is the latter worth buying and playing?
Also: if there's a design doc out there for Plague Inc or a similar game, I'd love to see it.
I've never played the original Plague Inc, so I can't tell you the difference from experience, but according to Wikipedia (https://en.wikipedia.org/wiki/Plague_Inc:_Evolved , isn't it amazing that it has its own Wikipedia page?):
The core game of Plague Inc: Evolved is the same as Plague Inc. - the player controls a plague which has infected patient zero. The player must infect and kill the whole world population by evolving the plague and adapting to various environments. However, there is a time pressure to complete the game before humans, the opponent, develop a cure for the plague. [...] It also includes all of the expansion packs from the original Plague Inc. game
Great game concept, though still with some untapped potential in my opinion. I keep getting inspired to replay it by the news right now, but we really have much, much more urgent things to attend to. Prepare, and help out, but stay safe!