The choice of lenses depends mainly on your Sphere power. It's wise to choose a high index if you need high power. The lens will be thinner and lighter if you choose a high index.
In general you might say that a lens with high index will improve the look of the eyeglasses (by being thinner) and better comfort (by being lighter). This applies in particular if you're very shortsighted or farsighted. It's not recommended to chose standard lenses for persons with high power, as this lenses will be too thick and heavy for the frame.
The example below shows how the thickness is affected by the different types of indexes.
If you're farsighted, which means you have ”plus” (+) power, the lens is thicker in the middle and thinner on the edges. If you're shortsighted, which means you have "minus" (-) power, the lens is thicker at the edge, and thinner in the middle.
In addition to your choice of index, the thickness of the lenses will also be affected by the power, PD and the size of the lens