Pop music wise the USA - Britney, Gaga, Miley, Ke$ha, Katy, Rihanna (even though she's from somewhere else she is very much based there) etc etc....whereas the UK has not got any good pop acts atm if you ask me.
Rock/Indie/Alternative wise UK all the way - Queen, Arctic Monkeys, Oasis, The Beatles, Led Zeppelin, Muse, Coldplay, Pet Shop Boys, The Rolling Stones, David Bowie, The Darkness, Kaiser Chiefs, Goldfrapp, Florence & the Machine >>>>>>>>>>>>>>
Mother Slayty Perrysa - Carriessa Underwoodsus - Kaceysus Muslay
Lykke Li - Ashley Monroe - Bring Me the Horizon
Mick Flannery - The Lumineers - LORDS BASTILLE
House of Cards - The Good Wife - Parks & Rec