I have the Scout. Totally love it. I am going to add a 120 to the 5.25 bay area for some added cooling. Kinda like what the 900 has. 900 is a nice case, but the lack of behind the mobo tray cable management turns me off of it. Plus the Scout has a black interior. Can't go wrong with that. But really, except for the cable management issue, either case is a great choice.
I would choose the Coolermaster Storm Scout. Ventilation, air flow, and cooling is very good. The case is 8.6 inches wide so it can easily handle the tall tower style cpu heatsink and cable management is good too. The case is 19.5 inches from front to back so it should be able to accomodate most of the longer video cards.
The Antec Nine Hundred is of the most popular cases with gamers. I haven't seen actual sales figures but I think it is Antec's all time best selling case.
Choosing a case if often a matter of personal preference. Buy the one you like.