you do know this question is anatomically impossible to answer? women generally have more tolerance for pain (mythbusters approved!) for just such reason.
a kick in the nads is a quick, sharp pain, but childbirth is hours of what i imagine is agony. maybe getting kicked in the balls every 10 minutes for hours can compare, but they are 2 very different kinds of pain as it is.
id go with childbirth....thinking about whether id like chinese water torture or a high-pressure fire hose to the face, im of the opinion elongated mild pain is generally worse than quick severe pain; and childbirth probably isnt all that mild anyway, if the screaming and crushing peoples hands is any indication.