I think it really depends on your field. If your interested in Latin American History, Stanford would be the best bet. They are shaping up that field to rival UChicago's LA field. If your interested in U.S. history, Stanford is losing all it's major U.S. historians - Camarillo, White, and Freedman.
If you're interested in Imperial history or empire, Oxford is better for the worldwide connections. Many scholars at my undergrad did their doctorates at Oxford
But unless the department is giving you funding, Stanford would be the better bet. The stipend is around 31k a year (26 +5k summer), and TA experience. Plus as an international student, it would look great to get degrees in different nations