Recently I've been reading a LOT of people say that Asylum is better than City, but like Zersch I disagree. Both are great but City was on another level for me. It's the first open-world game I played that actually made me want to explore and do all the side-missions. I couldn't wait to see what Batman villain would show up next, and it was really cool to see how they all intertwined (like the story with Penguin and Mr. Freeze). And the boss fights are way better. Only thing I missed was Scarecrow, but he got represented pretty well in Asylum already.
You can glide around the city all you want. Eventually you get a power-up where you do a sort of dive-bomb. Once you get enough acceleration you can pull back on the control stick and it's pretty much like flying. It's a glorious feeling.
And I enjoy games that are short, especially games that manage to be as compelling a ride as this one. In fact, knowing City is significantly bigger gives me pause, even though I've owned it for some 8 months
It's bigger in the size of the world but the game can be as long as you want it to be. If you only focus on the main quest it's right around the same length as Asylum I believe.