I'm a moderately devout Christian. And I agree with the American Atheists that the WTC cross has no place at the memorial.
We should not be privileging Christianity, or any particular religion, in public discourse. We should not be privileging theism over atheism and agnosticism.
I practice my religion in private, at home and in my church. I don't ram it down people's throats. Crosses belong in churches, not in the public square.
The United States is explicitly a secular nation. We must keep it that way, so that all people are free to practice their particular beliefs (theistic or not) as befits them.
Turning the US into a so-called "Christian" nation will grievously harm *all* belief / non-belief. If the Christianists think they are somehow "saving" this country or Christianity by trying to impose their extremist theocratic views on us, they had better think again, because what are they doing but seeking to impose "sharia"* with a cross stamped on it? If they get their way, will they dictate their particular brand of Christianism on my church? If they do, they will lose me, for I will not bow down to their theocracy and will very willingly become unchurched.
Having symbols of other religions and of atheism at the memorial will be better than the current situation. But rather than getting into squabbles about "your symbol is bigger than mine, so I'm SUING!!!111!!!!elevensies!", better that there should not be religious symbols at the memorial. We should be honoring the memories of all people that died – atheists, agnostics, Muslims, Jews, Hindus, Buddhists, Wiccans, people of other faiths and beliefs – and not just Christians.
* "sharia" in quotes, because the Christianists have absolutely no understanding of sharia.