ya i don't know why they do that, so if you have the room i would just grab the 320 and it leaves room for either more stuff in the loop or quieters fans, the ones i put in that list where 88FM high speed 40DB fans.
the setup i plan to buy will have 6 of those fans on a fan controller so when i am gaming or bench marking i can put it at full speed for best results. also the push and pull setup helps more too.