Personally, I'd say to NOT rebuild the WTC.
Leave it as a memorial: to those who died; to those who died trying to save/rescue the victims; and to the horrors of conflict, that we as a race might learn from it and the events that led to the carnage, and hopefully to never repeat them.
The wreckage can serve as a memorial in its own way - hopefully as a standard for peace and freedom, much as the Statue of Liberty does a couple of miles away.
Don't rebuild it. What it stands for will never be forgotten if left alone.
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"...and more controversial than Oolon Colluphid's trilogy of philosophical blockbusters Where God went Wrong, Some More of God's Greatest Mistakes, and Who is this God Person Anyway?"
- The Hitch Hiker's Guide to the Galaxy, Douglas Adams (dec.)