Borofloat is a bit tougher than UCL. Borofloat is a harder glass than the UCL, so it may be a bit more durable. However, UCL has more light transmittance. The lens above should be a more than adequate replacement for the plastic stock lens since it won't scratch up to ruin the beam, it will provide more light transmission, handle the heat of extended runtimes better and it's got a rubber gasket around the edges to help with shock if you drop the light.
I have one in my SL-20 and am happy with it. Huge improvement over the plastic one.
Of course any time you have glass it may crack if you drop it onto a hard surface. Just keep the plastic one around in the unlikely event of this happening until you can get a new one.