First of all its very hard to compare the two games directly. These two gigatnic RPG's are very different and this will make your decision harder.
Batman, being Batman, has tons of gadgets at his disposable again, this time though, you get to start off with some of the more basic ones, like a zip-line and graplehook to scale buildings faster. This isn't to say Geralt doesn't have anything at his disposal. His signs and potions are there to help your character though the game and you can even spec up your character into different skills, to make him suit your gamestyle.
Here is where Batman gets the upper hand, in its storyline, Witcher's tends to get boring at times, forcing you to explore the world at large and what a beautiful world it is. It makes a more pleasant sight for the eyes than Gotham's dark murky streets that the franchise is known for. Arkham Knight on the other hand has a story that will make you want to forget about, the frankly fun but annoying, AR challenges and riddler trophies dotted around Gotham.
This is the key to Batman's success ever since Arkham Asylum, not to say both of the original Witcher games haven't had an amazing story to tag along to the action, but Witcher 3's is so weak in places that Arkham's sinister tortmentor gets the winner here.
Combat in itself is also hard to call. Geralt's action packed, bomb fueled, sign induced combat is by far the most improved of the two. In the second game it was so hard to get ready for battles during the game as you needed to power of foresight or the patience to die repeatedly to learn the world around you. More streamlined and a little easier to learn with that rather brilliant tutorial for new players.
Although, a bit more up-to-date, Batman's isn't exactly as improved as Geralt's adventures, but makes subtle changes to the combat that give you more control and even introduces new ways to start fights with thugs.
Villains. These guys make the game, they make the story and the make the protagonist's attitude. Geralt (in his rather grumpy way) likes to dispatch everything that poses a problem, for a fee ofcourse. His foes consist of the wraiths, trolls, and anything else out of fantasy or mythology. These fantastical creatures are somewhat more fun to fight against, mainly because trophies they drop gove you even more loot and character bonuses.
Ofcourse, if you read the comics, you may still find Batman's arch nemisis' more engaging. Main difference is in grumpy Batman's world, he never kills a superviallain, instead locking them up for justice.
Boss battles are even harder to choose between because we know Rocksteady are the king's and Queen's of staying true to a character, even when you're punching them like Rocky punches meat in a butcher's shop. Witcher 3's (more so against monsters in Contracts) are a little more notty gritty and learning the best way to take down the creature before moaning he diesn't get paid enough. Rocksteady do seem to be able to create a feeling of excitement before the big setpiece of a big chapter finale than CD Projekt Red.
In short, it will be hard to choose.
Witcher's open world will be more interesting to explore than Gotham, mainly because they've really scaled up the amount of free space you have than in preivous games. If you're a fan of a challenge (a real challenge) this is also the game for you. Death March will punish you for tiny mistakes and it will punish you hard. The combat becomes a lot harder and it also becomes harder to find health.
Batman's a game we know very well, the story is as strong as ever and the combat has been stipped down to City's masterpiece and streamlined a little, still iffy in places but very strong. It is also a game that will make you want to find the riddler triphies, but won't try to force you away from the storyline either.