Your going to get lots of opinions for and against both. There are so many threads that already cover this debate that you are best just doing a forum search for "sand in run" and "wood chips".
It really depends on your individual circumstances so the more you read about others experienced with the two materials, the better you'll be able to judge for yourself which is best for you.
I have sand in mine, it keeps the smell down, and easy enough to rack it out; chips and straw make it out from the coop but seem to make more of a mess than anything else.