EU4 is currently somewhat in flux with relatively short patch cycles. If you don't mind that and can deal with choosing a version to play (that includes the technical aptitude to copy the game folder and run the game outside of Steam), then go ahead and buy EU4, it has quite a few improvements compared to DW (some of them arguable, e.g. I don't think the trade system is actually much better, it's just different). If you prefer stability you might want to stick with DW for now.