I think many of the posts here are way off. In my experience there is no general preference for male or female vocalists per se although some people may for their own reasons prefer certain sexes singing and trends seem to follow whatever sex happens to be in fashion at any particular time. OK the pop charts currently are dominated by females but that trend changes on a whim. Nest month it could be different Michael Buble and Justin Beibers could be the IN THING. There is no "better than" as Polly tries to make out issue either....I have heard great vocalists from both sexes and some pretty dud ones as well. My own preference of rock music is dominated by male singers BUT that does not mean that there are no great female rock singers. I know many.
On a personal note I hate whiny nasally female voices that seem to be in vogue at the moment......but that is just my own preference. Many people like that kind of sound. It is all down to personal taste.

ps I just used Buble and Beibers in the same sentence. WOW must be slipping. Comparing these two is like comparing Sinatra with the drunk guy who grabs the mic at a Saturday night karaoke.