They are both so varied, US probably even more so, that you can't really say. US weakness included schools being substantially funded by local taxation, so schools in poor areas end up a lot worse than ones in wealthy areas even before PTA type fundraising. But they end up with some of the same struggles to get into high schools that the UK has, though larger proportion of the country has only one school anywhere near.
I think in the official International comparisons that the UK comes out a bit ahead. You don't even have one education system in UK, though: contrast NI with England vs. Scotland or devolved Wales, too.
I am struggling with this question myself; there are things I love about DC English education but other things that horrify me.
I went to a very good state school in the US and was pretty horrified by the state options available to us in London by comparison. Overcrowded, poor facilities, no G+T, low expectations etc. But, the idies my DCs attend are better than the state school I went to. My mother actually looked at private schools for my sister and me at one point and decided against. They didn't offer as many AP classes or languages as the high school. Mind you, the tax base issue is what determines how good the schools are on so many levels. It was a very affluent school district. It might be interesting to see if things have changed from the point of view of teacher quality. In my childhood, the schools were staffed by Ivy educated women who had fewer career choices other than teaching (like my mother).