People who knit more loosely typically prefer wood. But if you normally knit with metal, you may not have any problems with them slipping out. Some people who knit with metal may knit a knit on the looser side, so the dpns may slip out. But I'm a tight knitter, and with the dozens of projects I've used metal dpns for, I've had the needle slip out maybe twice ever. And that was always just in the first couple stitches on a new needle, which could have easily been avoided had I been paying attention enough to knit a few extra stitches before letting go. lol But I knit tightly.
If you're really worried about it, I think the safest bet would be wood. The worst problem is you'd get irritated that you can't knit as quickly as with metal (that's the problem I have). However, that'd be better than your needles dropping out all over the place.
My ultimate suggestion is to get both and try them out for yourself! Buy wooden ones first if you want the safest best, but the next time, try some metal.