It's not quite that simple, though. According to this review by Bit-tech http://www.bit-tech.net/hardware/cases/2010/08/04/nzxt-phantom-case-review/2 The Phantom actually has better airflow -for the CPU.- That is, the CPU temps were better in the Phantom than in the HAF X. The HAF gets better GPU temps, I believe it has a special ducting just for that purpose.
That said, I'd say they're about the same when it comes to airflow, especially if you add a front 140mm to the 4 it comes with.
Aesthetics are all up to you, I own a Phantom, and wouldn't trade it for ten HAFs. The Coolermaster cases are hideous. The Phantom is also quite a bit cheaper here in the states. 200 USD as opposed to 110-140 USD. It's a no brainer if you like the NZXTs looks.
HAF X! I was comparing most of the cases on the market and the HAF X just has the extras which make it special, great quality, great looking and massive!!!