Hossa
Registered User
They've been around a lot longer, giving them a wider fan base. Even just looking at their new albums, Metallica outsold her by 100k in the first week of availability (280k to 170k, including streaming). I highly doubt she can sell out stadiums consistently world-wide like they can.
On some level that's a question of how to identify a proxy indicator for number of fans, because it's not like registering membership of a political party. For example, selling out stadiums is one thing, but Gaga has almost ten times the number of Instagram followers. Sales of a recent record are also useful, but then again if you look at most downloaded songs in Itunes, clearly that's more Gaga's territory.
Anyway, worldwide implies beyond the US and the west, so for any of that to be truly indicative you'd have to break it down a bit and it's really not important enough to bother with that, especially as their target demographics probably don't overlap too much. But I can only speak to the eye test of living in and travelling throughout Asia over the past 3-4 years, and Gaga's prominence here is remarkable. I can't imagine Metallica comes close in terms of popularity in this part of the world, but I could be wrong.