White.
Whilst some dark ones look 'cooler' - you have to cater for people with colour blindness and eyesight difficulties, so if your presentation is, say.. 10 feet away, some people might strain to read light text on a dark background.
Clarity and boringness should sadly win over 'coolness' and whilst I'd actually agree that in most cases, dark looks nicer - You're less likely to end up with an annoyed audience as they'll probably be able to read the text on your presentation if you stick to white.