This question will tie into what people like in general. People who are more into metal and rock will probably lean more into UK while people who are into pop culture might lean more into US. I like a lot of the music that is in nowadays and I like korean hip hop in general which was influenced by American media so I'm going to say US.
Depends what you like, really. If you like rock music you could make a strong case for either country. I'm not a huge rock fan myself, I prefer rap, and in that genre it's no contest. Only UK rappers I even know are the S. A. S. guys that Dipset signed, and I never quite got used to the accents. http://youtu.be/zaagW0S-5l0 I can't lie though, "And it's over, doja, when I flip and put the Razor to your face, I ain't talkin' 'bout a Motorola" is a hot line, haha, at least in '06. cdn2.gsmarena.com/vv/pics/motorola/mov3_00.jpg
UK made some pretty iconic artists. It doesn't matter if they only became superstar famous after reaching the US - that's only a testament to how good they are. Half my playlist are artists from the UK. The other half is shit like Jrock and Kpop. There's actually very little from the US. This also depends on who you listen to though.
Seeing how America has been the most dominating nation in pop culture and in producing music for the past century and seeing how many UK genres are based off of American music then America definitely has the edge.
In terms of mainstream relevance? American music wins
In terms of Quality? Highly subjective since both countries specialize in different genres. So it really depends on the taste of the individual.
In terms of Quantity? America in a landslide. (But that's to be expected seeing how America is 5 times larger then the UK in population and 40 times larger in terms of landmass size.)
Me personally, since America had Michael Jackson... they win.
every bit of music we enjoy from the late 50's on up to current day all sprang from American blues before it, all of it. keith richards and paul mc cartney know it and would tell you so as well, even jimi dug up out of his grave would tell you the same. no American blues means no British invasion, which changes history dramatically.
i love the British invasion (pink Floyd, the kinks, the beatles, rolling stones, led zeppelin, van Morrison, etc)... but even those brits admit that without the America blues and folk there wouldn't be most of the British music of the mid to late 60s
The UK might have had an edge over the US for rock music in the 60s and 70s, and they've had some major pop acts in the last decade as well, but when you look at blues, country, jazz, funk, R&B, pop, hip hop? Hell, even film composition and musical theater.
Ahh as a Brit in the states I'll definitely say UK the yanks didn't appreciate blues and rnb us Brits polished it and sent it back! The black bluesman had to come to England to get discovered. Hendrix was a nobody in American eyes but in England he became a rock god. Beatles Stones Zep need I continue?
None of the above, I think Germany, Sweden, Ireland, Netherlands and Belgium have the best music.
Have to go with Germany or Austria. Hard to top Bach or Mozart.
The Beatles were born in the u. k. But didn't eco me world famous until they hit America. Def leppard was born in u. k. But didn't get world famous until they hit America, black sabbath was born in u. k. But didn't get world famous until they hit America, need I say more?