density of petrol  

In theory,the lighter the petrol,the better.For example,gasoline is the lightest among all oil products.But i am not saying that the best is the most suitable.When buying petrol,you should know the type of your engine,how it works with different petrol,you should choose the kind it works best with.I think you may consult the gas station when you buy.
Have a good day!

well, i think that we should buy the one with more density!

My knowledge is somewhat rusty in this field, though I studied science. Petrol in general is lighter than water. I guess all petrol vendors offer petrol that is slightly modified by additives, in order that certain benefits are derived. Even then petrol remains lighter than water. It may be very difficult to make out the difference in densities. It is the so 'Octane' value which is more important. But choice will have to take into consideration the vehicle engine design and capability in handling high octane fuels.