You're there to help your girlfriend. There's next to nothing you can do to help her with the birth itself unless you happen to be a midwife(*) or suitably trained doctor (and likely not much then - she'll be doing the hard work), so you're almost certainly more use to her by her head, where she can see you. It's easier to be supportive of someone when you're where their mind and main senses are.
But really, it's between you and her rather than between you and us. It doesn't get much more personal. What do you want? What does she want?
* You would be a midwife in the literal meaning of the word, but I'm talking about a trained midwife.
EDIT: Out of interest, does anyone have direct, proven knowledge of a woman breaking someone's finger while giving birth because of the strength of her grip? Maybe it's an urban myth, but it seems like a possibility. People can have a hell of a grip in extreme circumstances and fingers aren't that strong.