That depends (a lot) on the displaying and viewing conditions; are your slides projected in a dimly-lit room, or are the lights on? Readability of your slides is affected by that.

Although slides often don't contain much information that needs to be read in full detail, I find code examples to be better readable in a dark font on a lighter background.

Sometimes the conference gives some advice regarding the slides (dark text on light backgrounds or vice versa), since they'll know / set the room lighting conditions.