Short answer : Composite. Composite structures are generally preferred for buildings with at least ten floors when looked upon from a cost point of view. But just generalizing this would be too vague. A lot should depend on the particular project at hand. Though, composite wins hands down in terms of project completion time.
Long answer : You can do a short analysis yourself. Since a slight difference in cost can also be an important factor, one can do some calculations by first modeling the building in ETABS or SAP. Then, you can calculate the quantity of materials needed for both approaches. You can multiply by the rates to get your material cost. Usually, composite beams and slabs are cut cheaper than RCC, and the columns are at par in both the methods. Then, you can use project time calculation software and henceforth deduce labor, machinery, electricity costs, etc. Also, composite buildings save both time and material on foundations. You can add up to find out simple total costs. If the difference is huge, you need not care about miscellaneous costs generalizing them to come within the difference, and you have your cheaper method.
I once read that for a simple B+G+20 tower, the composite method must take only 45% the time and around 80% of the materials as compared to the RCC method. The paper had carried out an exhaustive cost analysis, as mentioned above.