Libs: If having civilian trials for the planners of 9-11 is the right thing to do then why is AG Holder?

Although I read several reports on this decision, none of them included ysis of how the determination was made. Frankly, it kind of surprises me, since it is harder to prove a conspiracy charge in the military than in a civilian court. But it would seem to me that just getting them tried, whichever system we use, is the most important thing.