Since there is quite a bit of real estate available, I see no reason we can't do both.
Build some kind of border marker around the whole area that shows where the WTC complex was,along with plaques explaining what happened. Maybe pictures around the perimeter that show what the towers looked like from that angle. Inside that, build the new office buildings, but not as big as the WTC, since we also need some of that space for the memorial. Also leave more room between them for greenspace and pedestrian traffic. Maybe wide garden walkways. And inside somewhere (center?), build a 1 to 3 acre memorial park, monument, reflecting pool, etc. The people affected need somewhere to greive and reflect that looks and feels sacred, and that can't be done well in an office lobby. Some of these people will never receive any remains of their loved ones, and this place will be the only attachment they can have. Ground Zero IS their cemetery.