It's actually an upgraded version of the same engine, and Path of Exile 1 is getting a lot of the new tech backported.
It was originally supposed to replace Path of Exile 1, but now they are going to exist together.
2 will be more refined and slower, with a brand new story and 6 new acts. Should be a fun time.
I didn't enjoy the first one, I am enjoying the second one a lot more. It's not perfect, but the stories are generally better and things feel more polished. It's still an old-school JRPG though.
Personally I much preferred Lies of P.
Lords of the Fallen is sitting at 55% positive reviews on Steam. Lies of P is sitting at a nice comfortable 89% positive reviews.
Lords of the Fallen had a pretty troubled development cycle (changing studios 3 times) and it has some problems.
I played through the campaign, then got bored pretty much immediately after. Switched back to Path of Exile and made 2 characters this league. Having a blast.