I encountered the same dilemma few years back, so i'll share with you what i did. First of all, ask yourself how much money are you willing to spend. EU4 has many DLCs (14) and even on discount you need to spend at least 50-60 Euros to get the base game with its core DLCs ( 3-4 Core DLCs ). HOI IV is still a young game, it has 3 DLCs with another big one coming soon (40-50 to buy base game with all DLCs on discount). Secondly, unless you have a big interest on the WW2 period, EU4 is the way to go cause it offers replayability with a vast amount of nations and never getting the feeling that you have the same goal in each of your games. Every path you take is unique and most of the time will lead to a different outcome. One could argue that EU4 on its current state can stand as a complete game while HOI still has a long way to go. On the other hand, HOI's "hardest" task is to conquer the world and repeat with different nations. So there is not much replayability in HOI after a while. Last but not least both games offer a big variety of mods that change the vanilla game and will give you a different experience.
EDIT: Eventually you can get most PDX games cause you can buy them on discount in really low prices.
