I would bet that they are using concrete because the raw materials are easier to obtain. Steel costs are real high right now.
I can't imagine it will be concrete all the way up. If so, that's going to be one heavy sucker.
If anything the steel would be on the bottom with the concrete on the top. When construction methods are mixed it is very often done this way. While Steel costs are high, so are concrete costs. Usually Steel is cheaper but you build concrete because in areas where you have height limits, you can actually get more floors with concrete than with Steel. This shoudn't be an issue in this case. Most office towers are constructed from steel as you can engineer larger and more open floor plates.
In this building you can see that they have structural elements every twenty feet or so cutting the through the enitre floors with cutouts for pass-thrus. Not a very flexible design for the end-users.