Depends what you're after I think. If you are looking to showcase your video to the largest possible audience then there is no doubt youtube is king. On the other hand with such a large user base it can be difficult to stand out from the crowd. As swoosh said YT is going 1080p shortly and it's processing is already much faster than vimeo unless you are a paid member.
Lastly almost everyone knows what youtube is. If you say "hey go check this video out on vimeo" probably 95/100 people on the street would go "on what?"