Sacred Haven of Debauchery

Okay, I'll try, but it will be quick, and I might forget something. Roughly like this:
The levels are longer, the enemies are more diverse, and there is a wider range of mechanics for the player's character (due to the class system). You can create your own party from scratch. More combat-focused. Replayability leaves something to be desired. The Fortress system is poorly thought out and ultimately useless. The characters are average, but each has their own side quest. It's also easier to assemble a balanced party. More hardcore. More linear. The world doesn't stand out much compared to others. There are practically no memorable NPCs (except for the antagonist, who is memorable for his stupidity).
In Tyranny, there is more replayability, but it is an order of magnitude shorter. There are a number of interesting mechanics (for example, the implementation of influence on factions and companions, although not without flaws), but there are no classes (the mechanics of the same Chanter are reserved for one single companion). The magic system is more interesting. The companions are more unusual and charming, but they lack personal quests (despite obvious hints). But it is difficult to assemble a balanced party (only one tank, two or three mages, a rogue, and a rogue-fighter). The plot is better (which isn't hard to achieve). Overall, it's easier than Pillars in terms of gameplay difficulty. More non-linearity (I counted four routes, and in some cases, they can be changed). The Fortress has gained meaning. There are quite a few memorable NPCs, although mostly they are faction leaders. The world is more interesting than Pillars, as the time and level of technology are closer to antiquity rather than the Middle Ages.

Added 5 minutes ago

I played Pillars, and after that, I didn't feel like playing Tyranny.

Actually, I had the same fears. But then I got into it and couldn't stop. Which is rare for me, and I even wanted to do a second playthrough right away. But then I got distracted by other things, and the desire faded.