You can disable smoothing for an object if you need the rest of your scene smoothed.

For objects that are not going to be smoothed, hard egdes save the most polygons. But if you smooth the object, the hard edges will split.

Bevelling creates more polygons, but the edges will stay sharp even if the object is smoothed.

Render shows three times the same object rendered with polygon smoothing on:

1. Soft edges

2. Hard edges

3 Bevelled edges