1. Superior melee combat
Far more moves, weapon options and it's just smoother and faster.
2. Superior ranged combat
The crossbow in The Witcher 3 is damn useless whereas the bows in Odyssey are quick, devasting and accurate.
3. More charming protagonist
Kassandra has the charm and grace to match the greatest stars of cinema. Geralt is as miserable and gruff as a fail son at Thanksgiving.
4. Larger map
The map in Odyssey is almost twice the size of The Witcher 3.
5. Better boats
You have a refugee dinghy in TW3. In Odyssey you have a proud ship of war fit for the high seas which can be customized and upgraded extensively.
6. Better story
TW3 you are basically just the backdrop to the story of Ciri and her MacGuffin magic and there is a little political nonsense with characters that barely have any introduction. In Odyssey you roam the greatest era of the greatest civilization while solving the mysteries of your past and influencing the future.
7. Better DLC
The Witcher 3 DLC you chase around a genie and a vampire. In Odyssey you get all new gameplay exploring the mystical land of Atlantis.
8. Better mounts
In odyssey you can customize your mount and your horse is named after the God Phobos, son of Ares. In The Witcher your horse is named after a roach.
9. Better exploration
There is more to see and do in Odyssey. You can climb every mountain and swim to the bottom of the ocean.
10. Better sex
In TW3 you can romance Triss the rat exterminator or Yennifer who takes every opportunity to emasculate poor Geralt while simultaneously being clingy and a shrew. In Odyssey Kassandra bangs whoever she wants with wild abandon.
11. Better Scaling
In TW3 enemies and quests were a set level so therefore were often too easy and gave awful rewards. In Odyssey enemies and rewards scale.
12. Better Upgrades
Weapons and armor can be fully upgraded with many more options and transmog in Odyssey.
13. Better crafting
Instead of making useless "dedoctions" like Witcher 3 crafting in Odyssey focuses on useful things like weapons and ship upgrades.The supplies are more universal and there are more options for obtaining them.
14. Better graphics and performance
Not really debatable. Everything is just that much more beautiful in Odyssey.
15. Better tracking
Instead of following miles of trails and scent marks like in TW3 in Odyssey you follow clues and geography.
16. More epic battles
If you want you can conquer and defeat every army in the Aegean in Odyssey. In TW3 you fight at most a few guys at a time.
17. More dangerous enemies
Start trouble in Odyssey and an endless wave of mercenaries will be sent to track you down and put an end to you. In TW3 your actions have no consequences outside of story beats and a few worthless guards being sent after you.
18. Better stealth
In Odyssey you can become the ultimate assassin slipping silently into a camp and deftly murdering every soul like a whisper on the wind. In TW3 you stomp around clumsily and must fight your way through every situation.
19. Better antagonists
Instead of knocking off the Nazgul from Lord of the Rings like TW3 in Odyssey you pursue a dangerous cult that has infiltrated every level of society.
20. Better setting
The amazing world of ancient Greece is brought to life brilliantly in Odyssey. TW3 has some generic castles, fields and swamps.