Cng or lpg conversion is better for zen with 35km per day runing???

i have got a zen petrol(2001 model),i am getting a mileage of 10km/litre & my average running is 35km per day. i am interested in converting my car to cng/lpg. Can u advice me which conversion is preferrable according to my requirment. Please mention pro & cons of each type of conversion along with cost