For taking good finish of concrete slab, you may either use steel shuttering or ply wood shuttering. In steel shuttering, if steel plates have smooth surface without any dent or deposit of mortar and are properly placed in horizontal plan i.e. perfect in tiny squares and in straight edge then this will give best finishing. You would need to use the water controlled concrete mix and the slab should also be well vibrated. For making it even more smooth, one should place the plate in true plan and join the edges perfectly. After this the plates should be cleaned and oil should be applied on the surface.
Once the oil is applied, then a paper tape should be pasted on the joint of the plates. Care should be taken that steel bars are placed gently and proper care should be taken at the time of laying the concrete. Proper cover supports should be given under the steel bars and no disturbance should be done by labor when they walk on plates.
Also, the mix should be homogeneous i.e.the water contents of the mix should be within the limit
In most govt. building where steel shuttering pattern is recommended by the architect, than you can see that this type of finish is available.
Shuttering using the shuttering ply wood is also good and gives good finish. Cost of this shuttering process is more. In steel shuttering plates the plates do not get damaged so the material can be used multiple times so rent of such material is less. However, the plywood gets damaged in the shuttering process and can only be reused a maximum of 15 to 20 times only if good care is taken for the material.
So, for an individual home construction plywood would be expensive unless your contractor can arrange plywood shuttering material.
Our experience is that even after using expensive shuttering material such as plywood if the labor does not take care while laying the concrete, one may end up requiring plaster on the surface. If for industrial buildings where an absolute smooth finish is not a prime concern or if you intend to cover the ceiling with false ceiling then cost of plaster can be avoided.
As you stated for lean concrete and plaster on wooden planks, I am in view that you should apply white wash or apply oil on plaster surface so that the concrete will not stick on the plastered surface.