A reinforced liner is built to resist punishment, like a radial tire, while a rubber sheet is essentially an unreinforced inner tube. When an innertube is inflated, the rubber stretches and thins until it's vulnerable to a nail or even a sharp rock.
Like that radial tire's steel belts, a reinforced liner has a high-strength internal scrim that gives the material exceptional resistance from punctures or tears. When preventing leaks in your pond is a priority, a reinforced liner is your best solution.
