I'd have to say that the risk level would be the same whether your partner is male or female. Anal sex by its very nature involves greater skin to skin contact than vaginal sex, and many microscopic tears will be sustained on both the active partner's penis and along the passive partner's anus.

However, I note that in reality there may actually be a difference in transmission rates of HIV from passive male anal sex partners as compared to passive female anal sex partners - but no one is gonna be quick to tell us about it. No sane medical person is going to ruin their career and life by publicly announcing such a thing, as they would be accused of indirectly promoting an unsafe sex activity by suggesting that one activity was less riskier than the other.

We had a court case in Australia recently where a man was accused of intentionally putting his female partner's life at risk by having unprotected vaginal sex with her without telling her he was HIV+. A group of doctors gave evidence in the man's defence that HIV could only be transmitted through anal sex, and that it was a myth that vaginal sex could transmit HIV. The doctors were roundly condemned as loonies by the medical community.

But still, detailed and publicly available research and statistics seem to be fairly rubbery in this whole subject matter.