Xanathar is a great book for players. Almost all subclasses have solid concepts that are easy to grasp and make work in nearly every setting, even some no-brainers that could have belonged to the PHB like Swashbuckler, Grave/Forge Cleric, Storm/Shadow Sorcerer... altogether these almost doubled the amount of subclasses in the PHB making the game feel well-rounded. Spells were also good albeit reprinted, feats could have been better but ok. The DM part is more hit-and-miss and it felt like a random bunch of topics with no cohesive sense; the bigger sections are really good (tools, traps, downtime, minor magic items) but the small interludes (knots, falling rules, identifying spells) really feel unnecessary and potentially irritating if you disagree with the ruling... practically they feel like unwanted sage advice articles you didn't need and actually made you pay for. Worst part of the book are the frankly offensive tables of real-life names (English names, seriously?).
Tasha is a problematic book... The majority of subclasses are fringe concepts, hard to integrate in fantasy settings at large, doubtedly playable more than once, and occasionally even seem associated to a plain wrong base class (why is the Swarmkeeper a Ranger instead of a Druid?). The book contains psionics in a form that might have won the UA polls but I really don't think it hit the nail, in fact there are people still asking when do we finally get psionics in 5e... it's there and some didn't even notice. Then there's the issue with the free character boosts, "optional" yeah right... they put the DM in a difficult spot. The alternative class features is the only very good idea of the book without question.
Overall Xanathar beats Tasha 90%-10% for me.