i dont see why you wouldnt use smooth shading everytime due to it not increasing poly count?
You usually use a combination of both.
They are different calculations.
The easiest way to understand may be to take a mesh, like the monkey head, and make a copy of it so you have two.
Use smooth only on one and subsurf on the other.
If you can get what you want with smooth and get it to deform as it will need to for your use, just use smooth. You are right, no reason to add what is not needed.
If you are trying to make a realistic face you will come to appreciate the subsurf very quickly.
In the end, they are both just tools to get what you need. Giving both a test ride is going to teach you more than anyone can explain I think.
Indeed. It all depends on how much detail you want on your model. If low levels of detail are all that are needed, then often just smoothing is fine. as long as you have no sharp angles which smoothing will muddy up.
thank you so much tfrank and mike! getting excited about getting back into 3d!