It is hard for any student to get into both Harvard or Stanford. It doesn't matter if you are international or not.
Just by looking at sheer numbers, (Class of 2017) Harvard had 185 international students while Stanford had about 135 international students. Looks like Harvard is easier but if you look at yield and admission figures, it pretty much balances out.
However, for Financial Aid, Stanford is need blind for US students and need-aware for international students. Harvard, however is need-blind for both types of students. Thus, if you just consider financial aid, it is probably easier for you to get into Stanford as they know you will pay your own tuition and not eat into their funds.
That said, if you are good enough to get into Stanford, you will probably also be good enough to be admitted to Harvard and it would really boil down to luck, as to which school finds you to be a better match.
Note: I couldn't find the admit rate for solely the internationals. If anyone can, please comment on this answer.

simply... HARVARD