There are like 10 good european metal bands for each american band I can think of. Anyway, if you include South America and Canada, it goes to something like 5 to 1, but still a shitload more. So, Europe gets my vote.
Anyway, regarding Europe having more inhabitants than USA, i would think on it. I mean, Italy has got 60 millions inhabitants but only a single good band I can think of (novembre). There are no Spanish and no French bands I know, and that adds anoter 130 million people without good metal bands. So I think the whole Europe kinda balances Scandinavia, not the other way around.
And, anyway, Scandinavia would win against USA.