Check out this link i've been doing some research my opinion LPG is better than CNG, One reason i like is you can get a donut tank that can fit into your spare tyre space and don't loose any trunk space, Just you will have to run with no spare

but check out the video on ( How far can 60 litres of LPG run on a Toyota Hilux )
http://www.youtube.com/watch?v=QxoYkHZhV98