I don't see that either is objectively more offensive. Clearly "bitch" is more offensive if you're a woman and "bastard" is more offensive if you're a man. Apart from the original meanings, both have come to mean roughly "annoying person whom I greatly dislike of the [male/female] gender" (gender depending on the term, of course).

I've heard gay men call other gay men "bitch" -- not being polite either; it was a term of great opprobrium. So perhaps that term will eventually even lose its gender-specificity.

Alex