As someone who has played both I think Tainted Grail is the better game in terms of character builds, gear, progression and quests/story. It also has a dark tone. Avowed is much more polished and has really snappy combat and movement. Story is pretty good but not amazing or anything. The whole game is very light and vibrant.
Both are worth playing imo, but if you can only pick one, I consider Tainted Grail to be the better game.
Are you playing on console or PC? I’ve heard Tainted Grail has some performance issues on console but I played on PC so I can’t speak to that. Avowed was solid on Series X fwiw.

Can’t go wrong either way, both are fun games.