Now that I have the Air 540, I prefer my HAF XB. The AIR is nice and all, though I don't like all the plastic that was used. The window is nice, I thought about using it on the HAF XB if I can manage it some how. The layout is similar, though I would say the Air has a slightly nicer interior. It has the rubber grommets and stuff. Though with how I'm managing the HAF XB I'm planning on thee radiators, 2 x 240mm, and 1 360mm in the front. It will allow for more then enough cooling. The real thing that made me chose it was the metal construction over the plastic, it just feels more durable. I really thought for being 40 or 50 more then the HAF XB it would be of higher quality then it is. Not that it's bad, I just don't prefer it. I'll use it for my wifes build maybe. Also, the AIR would be hard to sit horizontal. As the PSU would then be forced down, against the ground. It could be done with some modding though. I thought about this as well, it would be cool.
The HAF XB is going to be my first watercooling build, and I'm diving head first into it with acrylic tubing. I've read a ton and done a lot of research, the guy who did the demon speeding had a ton of great ideas. I'm going to be using a few of them in this build. The Thermaltake case will be for an entire different build theme, I might go ROG or something with it, or military/industrial. I'm not sure yet, I just want some experience first.