The UK is better if you want to solely study economics intensely, but the liberal arts education in the US allows you to receive a more well-rounded education which may be more beneficial in terms of getting jobs within the finance sector.

I would also recommend the US over the UK if your interested in working in finance as the industry is much larger in the US than the UK. If you want to be an economist, or work more on academia, the UK could be a good option due to the focused and thorough nature of the British Educational system.