cost wise rattle cans would be cheaper for short runs, when you factor in the cost of protective equipment as you would at least need to use a 2k lacquer for the tank to guard against petrol spillages, you could use a clear 2k rattle can but would still need a good respirator, paints are cheaper when bought in bulk, but shelf life is a problem if not used for any length of time, tbh unless you intend to do it as full on business rattle cans will almost certainly be cheaper for one off or small production runs, and rattle cans can be custom made to any colour now which does save the hassle of mixing it yourself, then there is the fact of very little clean up with cans as opposed to a gun, which will need a certain amout of clean up after every use