IMO, nothing except a memorial should be built there. Whether it's a park with a small twin tower statue or what have you, nothing else should go there.
Someone wanting to build a mosque there has an ulterior motive, and I take it as a slap in the face. It's extra salt in the would that a place of worship that is the same religion a bunch of individuals pervert into terrorism is going to be built there.
Why a mosque there? Just good real estate? There's something else going on there.
Many different races and religions of people died in the 9/11 attacks. Why is it just one religion is going to be catered to? This just isn't right.