bit varying(512) vs bytea(64)

I have N-bit data pairs. I want to write a c-language function
which compares bits set in each. N is typically 512, but could be other.
I can store this as bit varying(512) or bytea(64). I can't decide which.
Here are the questions that concern me.
1) will each take the same storage?
2) can I pass bit varying data to a c-language function? I can't find any
docs or examples of that.
3) are bit strings stored as actual bits or as character strings of 0 and 1?

Thanks,
TJ