Sorry, strace wins by a landslide. Decoding the flags to symbolic names is hugely useful. Decoding the struct sigaction is even more useful (since I can't even do that manually by grepping include files -- you've just given me a random pointer, completely useless).
Argument names, not so useful: that's basically just noise. I know the API of mmap, but I sure as heck don't know what the flags' raw integer values are!