I agree, you cant compare the 2 cases. The HAF X (which gets great reviews) retails at about £115 here and the Corsair 900d comes in at £300. Huge price differential. Also, the Corsair 900d is a Super Tower (Huge Case) - which can house 2 individual WC loops and rads, 3-4 Cards in SLI, Fans galore, etc.
IF the 900D case is withing your budget, definitely buy it. The reviews across have come in favourably and it will last you many years and house anything you could imagine building for many years to come!