The UK will be faster, but that does not mean it will necessarily be better. What are you goals post-PhD? Are trying to stay in Academia? If so, then the US is most likely better.
On the average, US PhD students graduate with more, and higher impact, publications than UK PhD students. The reason for this is that US PhD graduation is typically contingent upon producing two or three significant publications in your field. So you might have to stay for six or seven years, but you will have an established publication record when you leave.
As far as I am aware of, no such publication requirement exists for any UK university. UK PhD graduation is contingent upon completing a thesis and viva (defense) but often times the research within the thesis has not yet been published. There is typically a period after PhD graduation where UK students then focus on publishing.