Ok, as you are looking for opinions about the games, I will throw mine too. Listen to Stallion ! I'm more of a Marvel guy but having played both there is no comparison in my mind.
Batman 2 has no achievements on PC, a thing that in another case wouldn't have mattered to me but here the cheevos could have really pushed me to play more in the open world. Okay, carrot on a stick to push players to play an aspect of a game seems like a bad idea, but really, here I think it wouldn't have been a bad thing.
Oh and yeah, the open world in Batman 2 is relatively weak. Not bad but IIRC it's small and not particularly interesting.
But if we talk missions ? No comparison. Batman 2 has humor, epic, and what I subjectively felt were diverse and interesting levels.
Marvel has cheevos, a bigger map (as boring), but the thing is... I don't think I played a mission that wasn't a big long level of nothingness. God, that was painfully dull. But yeah, thinking about it there is surely more content, as there are little side levels to unlock in this one. Not bad, the side ones, not really stellar either, but in any case definitely not enough to save the game from being the boring one to me. I don't know how to say it, maybe it's just a writing thing, where I was entertained by Batman 2, Marvel kept surfing my Marvel vibe without really trying. It wasn't funny, it felt more like a checklist trying to maximize the number of references to the comic books, but lacking passion.
Let's put it this way, you present me the two games, I would think Batman 2 is a solid game on its own and Marvel a product. Not an abysmal product, I spent many hours playing it, but the spark was not there.