Definitely medium. You can fit into a small if you really want to at 31 with AK fit. Large will give you the pants around ankles look.
Are you refering to the older AK model pants with the Sig fit? The newer 2012 and up has the AK fit which is slimmer and shorter in the inseam I think. The medium does fit me well in the wait and legs but I think its just tad bit short. Does the stagger and cyclic pants fit the same?