In my mind, for a computer case anyway, looks and style take a backseat to cooling. It's just the way I view things. I know for a fact both the 900 and the 1200 give excellent cooling, but I have no idea about the HAF.
But seeing as you already have a 900, it's down to two choices then.
What it boils down to is what your putting inside of it. 1200 is good for people who put alot in their comp (aka double or triple SLI/Xfire, sound cards and alot of drives). It also has damn good cooling.
The 900 and HAF are more compact, and they also have good cooling. I'm sure double SLI would be no problem in either, but you can't pack as much customization and equipment into the case.
It basically boils down to a personal choice. What are you going to put in it? Is it going to be a gaming computer or some internet cruiser? What cooling option are you using for the CPU and the GPUs? Are you using water? air?
Alot of factors you need to take into consideration, so post what you're using and I'll be able to help out a bit more.