Question

which of these diffuse faster LPG or CNG​

Answer 1

CNG is generally cheaper than LPG. However, propane produces over twice the amount of energy than natural gas. So LPG could be more cost effective than natural gas depending upon local prices (which tend to fluctuate).

Answer 2

In one study it has been estimated that CNG vehicles produce around 20% less carbon monoxide than gasoline powered vehicles and 25% fewer greenhouse gases. ... So in case there is a gas leak the gas will diffuse rapidly