Which way are you leaning and why? Are you wanting to keep it really clean and willing to rake and scoop, or wanting more of a deep litter method type thing, where it's not "clean", but you don't have to mess with it?
I would think sand. The reason I believe is they have to learn first what is allowed to eat. Once they learn that wood chips are not for eating, then should be alright! Congrats on the chickies!
I would place paper towels over wood chips for atleast the first 4 days so that they learn to eat their food. Paper towels are easy to change out. Afterward, they would be on the woodchips. Igive them a frisbee full of sand to play in and dust bathe in. I love the chick stage, but, they just grow too fast.