They're dramatically different games.
PoE is a grand adventure with deep lore, a sullen tone to the world and story, prescribed characters, and a 'real-time-with-pause' battle system similar to what you would see out of an old PC DnD game from the late 90's - early 00's.
EviLore described it like this:
Pillars of Eternity is a real-time-with-pause tactical party-based fantasy role playing game, spanning two large hub cities and a vast expanse of wilderness locales and beyond. If youre unfamiliar with the subgenre or youre not a million years old like those of us who crowdfunded this game out of our massive old-fashioned Costanza wallets, think Dragon Age PCs strategic camera mode without all the suck. You control your party of heroes with RTS-style click and drag controls, pausing when necessary to queue up actions and abilities, then unpausing to see it all play out in glorious mayhem.
Pillars had a design goal of recapturing the look, feel, and uniqueness of the classic Infinity Engine computer RPGs (Baldurs Gate 1&2, Planescape: Torment, Icewind Dale 1&2) of fifteen years ago without compromising the game for broader audiences or console-friendly controls or anything else that would typically be necessary for a mass market product in 2015. Like the classic IE games, it uses prerendered backgrounds with hand-drawn elements for a timeless and beautiful look. Just look at those screenshots.
Instead of using a D&D ruleset or its Forgotten Realms setting, though, all of PoEs game systems have been built from scratch, along with a elaborate original fantasy setting: Eora, a world which delves deeply into the concept of souls.
.
Divinity: Original Sin is a much more wacky or goofy affair, fully turn based combat, fully voice acted, the combat is one of the strongest of any recent RPG, multiplayer co-op (2 players for D:OS1, 4 players for part 2) that allows another player to drop in at any time and even split up where one player can be in a fight while another is still in town adventuring, almost anything can be interacted with, features creative elemental combinations that frequently get used for environmental control, etc etc. The first Divinity is also on PS4, btw.