I think Eternal has the better gameplay and combat arenas, but as a "campaign" the first one flows better. It has better pacing and level design.
There was an interview with Marty about how in Eternal they always wanted the player to be engaged even when they're not in combat. This led to them adding sections like those tentacle hallways, poison water that you can't jump in, underwater swimming sections, etc., all of which I thought were genuinely terrible. Same goes with the expanded lore, story, and cut-scenes. Just really bland stuff.