ng gives you a greater supply than lpg
you do lose a little power on ng compared to lpg
ng may not be the best choice in an earthquake-prone area
+1 on the natural gas being a tough choice in an earthquake area - was one of the deciding factors for me to go with the PW.
I'd say though you're losing more than "a little power" comparing natural gas to propane: propane has over 2x the energy by volume.