That’s like asking if you want your million-dollar prize in hundreds or fifties. There is no best ISO for concerts since concerts have a full gamut of lighting. The ISO for a daytime concert in the stark Nevada desert is vastly different than taking photos at your dimly lit local dinner theater. There is no single setting, and ISO is a function of the camera, not the lens so find one that can be manually set on a wide level. In the digital age, we’re not locked into ISO as we were in the days of film.