If you can do it at Oxford I would definitely say do it there. It'll be faster, you get to experience in the UK, and Oxford is a really nice city and university.
1. Subject/Supervisor 2. Location and acceptance in your target industry 3. Time for completion. 4. Cost and funding.
1. Supervisor relationships can make a massive difference in a PhD. You will work ridiculously hard either way - but a good supervisor will make it feel like worthwhile while doing it. A bad one will make it feel like hell while doing it. Checkout other postgrads and post doctorates who have worked with the supervisor you are considering. What is the quality of the papers? How often did they publish? How long did ti take them to complete? Contact then and ask them about their experiences.
2. Oxford is a worldwide known university it will open doors, has a fantastic network and will not be a barrier to future US/international work. (This cannot be said about other good UK universities that are not as well known internationally). For the US - Places like Stanford, Princeton, Berkley and Caltech (+ other) are similar - they open doors and have ready-made networks that can help. When looking for a place look for the ecosystem - industrial and academic - to see which can support both your research and possibly your post-doctorate career.
3. Time for completion will be massively dependent on the area that you do and pressure/support to publish from your supervisor. In general a UK PhD may be a more condensed experience if fully funded and fully focused but that is but not always the case - it depends on how your PhD is financed and structured (also dependent on supervisor).
4. Economic stress is a big component of people not completing PhDs. Spend time working out all costs (cost of course, cost of living in possible location of your candidate universities) and what financing/funding is available. Even if the PhD research position is funded - it may not cover the costs of living in a specific location.