Doom 2016 and Eternal are both really good purchases imo. 2016's combat and level layouts really do feel more like a "doom if it was released today" kind of thing (in a good way), while Eternal's combat mechanics (re: lack of ammo) demand a more specific playstyle and level layouts are generally more linear. In a perfect world, I'd be able to back-port some of Eternal's improvements to 2016 and have a game perfect to my tastes, but oh well. I'd lean more towards 2016 if you're only getting one for now, and consider Eternal if you find yourself wanting more modern Doom.