They basically all give you enough protection if you just fall off, and not enough if you're hit, so pick the one that is cheapest/neatest-looking.
I like my visor; it keeps the sun out of my eyes for three minutes more than otherwise... but it's a nice 3 minutes! I wouldn't pay any extra money for it, though (and some people think it is totally uncool to wear a visor on a road bike... meh).