I'd say on average females have better voices, but the odds are against them. For one thing, popular music for female singers is unequivocally crap, even compared to male popular music; the female singers themselves are trained to whine, wheeze, moan, everything but actually sing; and the image problem [no rack, you're sacked] is really weighing on them.
Also, although males are on average worse singers, their very best can outdo any female. Lennon/McCartney, Simon and Garfunkel, etc, are not going to be outperformed by any woman singer I know of today.