HoI4 is further in its development process, as beta testers were chosen two months ago and there are no signs something similar will happen for Stellaris soon. But as far as I understood, there is quite some trouble with HoI Beta and not all chosen people are able to play the game properly, so Johan stated the games state as "still alpha" a week ago.
So if these problems persist for some time longer, Stellaris definetely catches up. Currently I wouldn't bet which one will be released first.