Arkham Asylum fan here. I prefer it purely for its atmosphere and focus. The single location, the way the Asylum shifts and changes throughout the game, the creepy, mysterious vibe, it's just done really, really well. Arkham City is a well done game, and many facets of it are superior to AA, but in my opinion the increased scale of the game is to its detriment. I found it harder to get immersed and pulled into what was going on with the game constantly throwing shallow side quests, challenges, tons of characters, cheesier dialogue and sequences (the conversations with Catwoman, dodging a shark in a flooded room, etc.) at me. It's like eating too much candy. I totally understand why they went in the direction they did but it wasn't for me.