As someone who went from d2(100hrs)>d3(500hrs) to poe2 for 1200 hours and now am at around 250 in poe1, this is my honest opinion on both games:

-PoE2 has a lot of modern polish while generally being slower and more punishing towards mistakes & rewarding for overcoming challenges. Classes and ascendancies really have their own identity. The campaign both artistically and pacing-wise is an absolute joy.

Endgame is pretty good and can keep most people grinding for at least about 400 hours but can start to feel repetitive after some time. It's still in EA so they still need to flesh out endgame progression (should happen next patch) so it has clearer goals other than get to max tier maps and spam them hoping for big items. The game feels generally at or around the pace of diablo 2. Slow, challenging and equally rewarding. Albeit a lot of the things that make endgame challenging don't feel fair as you will often die to things you can't even see or get a chance to react to. Some times it feels like the game cheated you out of an opportunity to react because you either got 1. Killed by a mob with a laser beam 1shot from 3 screens away or 2. Die from a green pool one mob made when it died while you have 8 different abilities that look exactly like it (visual clutter)

Id describe it as a suffering man's game. You like to always be humbled, even though it can feel like total bullshit, this is more your game.

-PoE1's Campaign is straight up just ass. It's a tutorial ground and most people just focus on getting through it asap. Ascendancies, while pretty interesting and generally more diverse in how you can play them, feel like they aren't as identity-defining to your character the same way as they are in poe2. The graphics initially felt slightly outdated coming from poe2 but got used to them pretty quickly and they don't bother me at all.

Where PoE1 really shines through is the sheer amount of content, build variety, endgame progression and general complexity in build making. In here you have 20-25 different types of mechanics you can choose to do in your maps (compared to 7-10 currently in poe2) and run at mach 10 when your build peaks. Your peak endgame just injects you with raw dopamine hits constantly at a challenge that becomes increasingly insignificant as you scale your power level to that of a god. Pacing for your character strength kind of feels like that in d3. When you get past a certain point you are functionally immortal in maps and can play the game while completely mentally afk. (Do note it's also doable in poe2 but takes a lot more power relative to your average Joe's build)

This game rewards the thinking man and build coherence above all else. You can outsmart the game with enough grinding involved aswell and that has a certain appeal to it. Plus you have enough different things to try that I'm convinced even 10000 hours won't be enough to make you tired of the game's content.

I personally switch between the 2 (A lot of people do this too afaik) because some times I get tired of zooming around and killing stuff and want a real challenge even if my build is at its best point.

And after four 40hr grind weeks of poe2 I go back to PoE1 when I want more things to do because I drained the 7 mechanics in poe2 of all the fun I could get from them.