All the loads that will act on the structure. Use load combinations as per code and make a structural analysis to determine how much load will be carried by your columns. There are lots of loads to consider so please consult a structural engineer.
Decking will provide a better finishing of the slab soffit if required by the owner/contractor. Less steel is required and less form supports and scaffoldings. Just place it on top with shear studs on top of a reinforced cap beam. Or you can try the concept of a hollow core slab placed on top of...