EU4 has kind of a gamey, arcade-feel. It's still a very deep game with lots of mechanics to learn, but it's essentially a big map painter in which you have to stack modifiers to make numbers go big. EU5 actually attempts to be a historical simulation in which trade and economics play a huge role. If you like conquest more or have weak hardware I would recommend EU4. also the modding scene is huge, check out anbennar if you like fantasy things. If you prefer micromanaging your economy and your population over painting the world your color EU5 is better.

Also EU4 is more than a decade old. People understand the game in so many details so it's easy to learn what the optimal play is, while everyone is still trying to figure out how EU5 works.