Remember, these brainiacs invented algebra, or so they claim.
A six-sided pentagon?
Shoot, even Ptolemy, four centuries before mohammed the pervert was born (sbuh), knew what a pentagon looks like. As do even the dullest fourth graders in any civilized country.
The Pentagon was the target in their last attack, and Al Queda has a track record with regard to "2nd attempts" (WTC first attacked in 1993). They also thump their chest about removing capability of communicating with deployed forces in Iraq.
A stadium makes more sense given their stated desire is to kill 100,000 Americans.
Of course, if they could do both - they would.