I love them both. I feel like they both need to be played on harder difficulties because they’re built for a pushy playstyle, especially Eternal which requires melee finishers for ammo drops even more than the '16 game already did.
'16 has more of a straightforward plot. The story is fine. The main NPC looks and sounds like James Spader’s Ultron, which thrills me. I love the Mars station design and wish the Hell levels were a bit more creative. Other than some mysterious hints at a connection between Doomguy and all the Hell stuff, '16 doesn’t bother much with lore.
Eternal takes everything good about '16 and gives it an espresso, some laughing gas, and a whole bunch of lore that might have been written by Tenacious D. It’s deeply silly, very hard and has some of the best game design I’ve ever seen. I don’t think one is better than the other; 2016 is more nostalgic, but Eternal is more ambitious. The only catch about Eternal’s ambition is that you really have to be on board, because there aren’t optional play styles — you play Eternal the way the devs tell you it’s supposed to be played.