HoI IV is the better game than Stellaris as HoI's problems lie in dumbed down AI and mechanics, Stellaris has problems that can only be fixed through dlc. I believe by the end of this year, we'll see mods on HoI IV fix a lot of the problems HoI is suffering.
Still I think you shouldn't get either one right now, maybe if the games are on sale but not right now. The games are borderline Early Access titles based on the bugs and problems with the AI. I highly recommend you look into Victoria 2 and Darkest Hour. Vicky 2 and DH have been fleshed out for years with many mods like Kaiserreich (Alternate-History for Darkest Hour), NWO (Cold War/ModernTimes mod for Vicky 2). I bought Stellaris and HoI IV and regret getting them on release and would have much rather bought them around October or so. Anyway good luck.