pacman uses libfetch. I just use pacman's libfetch because the output is nicer. I think some people claim that wget (or curl) can be faster. The real benefit is if you want to customize the download command. (Also, for a while there was a bug with libfetch that failed to close connections and after downloading a few packages, the new connections would be refused - but this has been resolved recently).
There's also a wrapper for pacman called powerpill which downloads files concurrently. I've always been happy with straight up pacman.