Without being a fly on the wall and with a little knowledge into the realities of metal fabrication.
1) The Model S and X are primarily aluminum structures. A few key components are steel and titanium.
2) Steel is cheaper to purchase but cost the same to fabricate with. So only raw materials is the cost factor.
3) it's common practice in the industry to build aluminum exterior panels for weight savings. Hoods and trunk lids are the most common.
4) Most aluminum cars and trucks have steel frames, such as the F-150 which has an aluminum cab and box.
5) Most cars today have aluminum in the structure, engines, drivetrain, suspension, and rims.
6) Tesla and the personal at the reveal stated that the Model 3 frame is steel and aluminum where benefits mattered most.
As its standard practice today, I suspect the Frame to be primarily high strength steel. With steel and aluminum used in different components. All the paneling, floor boards, door structures and front substructure will likely be Aluminum.
Aluminum is lighter then steel, but typically you need beefier structures then steel to accomplish the same result. Steel is there fore cheaper per component, but if your building large components the weight negatively effect the range of the Car.