Mileage: Mileage is one of the major positive for any car user for switching to LPG or CNG. There is close to 20% better mileage from a LPG/CNG powered car when compared to a petrol car. Moreover, considering the low price of either of these fuels, it is a double whammy for sure.
Price: As stated above, the price of both LPG and CNG is lower than petrol. In fact following the recent Rs.5 diesel price hike, both these fuels are now cheaper than diesel. There is a marginal difference of Rs. 3 to Rs.5 between LPG and CNG prices with CNG being the cheaper one.
Practicality: Carmakers are now offering cars with dual fuel technology. This means, even if your car runs out of gas, you can have a back up stock of petrol to take you to the closest fuel pump. In that way both CNG and LPG have an advantage. But the only difference here is the filling time. Filling a CNG car will take about 4 to five minutes, while on the other hand filling the LPG car takes just about half the time. People opting for CNG should be ready to wait for several minutes at fuel pumps.
Space: You will loose your precious luggage space if you switch your car from petrol to CNG or LPG. This is a given, the extent of luggage space differs. CNG requires a larger tank and this tank takes up the entire rear luggage space. LPG on the other hand being a liquid fuel can be accommodated in a smaller cylinder. Hence you will be left with more luggage space in an LPG car.
Drive and Handling: The addition of the gas cylinder in the rear will mean your car will be slightly lower. This will affect the car's suspension and the impact of bumps and potholes will be passed on to the occupants of the car. CNG cars need special attention as the CNG cylinder is heavier than LPG. A slight modification to the suspension is needed to accommodate the increased weight.
Verdict: Both LPG and CNG offer similar advantages. However, the availability of CNG is not as wide as LPG across the country. However, in cities where CNG is being sold it is more easily available than LPG. Both fuel offer a cheaper alternative to petrol. If performance is your priority, then you can choose LPG over CNG./ But CNG being a cheaper fuel among the two is ideal if you want to reduce costs.