[decimon]

I would let private developers decide what is the need for office space.

As for a memorial...well, I would favor something simple and dignified. This is a picture of the New York City Vietnam Veteran Memorial; the "Wall of Letters." It's not far from the WTC site, on Water Street. I can tell you that the effect of it is much stronger than would be any gaudy display. It's between three office buildings.

[stanz]

I agree that private developers should decide what is to be built on the site, too. There have been so many ideas generated for a memorial and the families of the victims should have input, but 9/11 happened to all of us. Something simple, perhaps just an iron tower which rises to the same height as the Twin Towers would stand out on the skyline as a fitting tribute. There would still be plenty of space for other habitable structures for offices, etc.
My personal choice would be to put back the towers and adjoining buildings exactly as they were and design a memorial much like the one you described.