I bought the thicker, single-layer type.  It seemed to me there would be
less leakage problems down the road.  Make sure you seal all the bolt holes
and check out the gasket around the opening.  I am pretty sure I got mine
from The Bus Depot because they were closer and the shipping was less.  The
Skylight Guy does offer more options on styles and colors.  The originals
were two-layers and  would crack and get water in between which soon became
stinky and leaked on your floor.

                                                                  Jay