The HAF series and Coolermaster cases in general are pretty good, though I find the 912 to be a bit bare on features. No USB 3, no tool-less bays, and only one more 5.25 and 2.5" bay compared to the Cougar.
I would go with the Cougar + a front 140mm fan. Works out cheaper than the HAF 912 and you get more features.
USB3 isnt essential, but a good feature to have. It offers increased bandwidth over USB2, so things like flash drives and external HDD's can deliver greater quantities of data without bottle-necking. That being said, only the faster flash drives would be restricted by USB2, and I doubt by much. The device would also have a USB3 connector to take advantage of the extra bandwidth.