Your balloon would have to be about twice as large to compensate for the higher density of methane (.656, according to google)

I also am not sure that would be much safer than using hydrogen. Methane ignites a bit harder and would initially burn a lot slower (hydrogen has a much higher upper explosive limit (https://en.wikipedia.org/wiki/Flammability_limit), so it would need to disperse more for the gas at the center of the balloon to ignite), but _any_ fire near the surface of the balloon eventually would be catastrophic, and putting fire extinguishers everywhere isn't an option because of their weight, in combination with the huge area of your balloon)