I am replacing a shower on a cement slab. Is it best to replace the pan with a prefab pan or traditional cement pan with liner?

All depends upon the look you want. Preformed could be plastics or fiberglass or solid surface like corian and are the base you see. There are preformed bases for tile such as Tile-redi, Schluter Kerdi or Noble's new one (can't remember the name) or you could built a traditional sloped mud bed-liner-setting bed tile pan