In general, in ETABS V8 it is better to define a slab as shell. Draw a single slab for the whole floor and then assign automatic meshing (1.00 mt x 1.00 mt shell elements is usually enough) to assure proper load transfer to beams and walls.
Will assigning it shell lead to high out-of-plane bending of the slab and hence reduction of total lateral deflection at top? I tried to model a typ. bldg. with both ways only for lateral loading. Results showed increase in deflection when slabs assigned membrane rather than shells.
I really have not done the comparison, but it makes sense to find smaller deflections at the top with shell slabs. Any way, real slabs do have out of plane behavior so the shell approach is more realistic.
