If paying for some/all of college is important to you, and the 529 plan is the one your state offers and you get some kind of state tax benefit? 529 by a mile. The tax break and tax-free growth will be a boon to your 'stash's tax efficiency.
If you want more flexibility on how you use the money and / or your state doesn't offer any tax benefit for their 529 plan, go with the taxable index fund.