I recommend the Roth, unless there are significant savings on state income tax from using the 529.
529 accounts are always assessed as parental assets, rather than student assets, so your financial aid calculations award is penalized less. Colleges want to make the student broke first, and then parents broke second. But it doesn't really matter if you're planning to spend the money on college anyway, because you're going to spend every dollar in that account either way.
The Roth IRA is a retirement account, and is not assessed for financial aid calculations at all. It's an excluded asset.
Roth IRA accounts can be tapped for college expenses at any time, without paying taxes or penalties. The problem with using them for college is that the annual contribution limits are pretty small, and they cut into your ability to save for your own retirement if you use them for college.
If the kids have earned income, they can set up their own Roth IRA in their own name, which can also be used for college expenses at any time without paying taxes or penalties, and you can fund it up to the amount of earned income they have, or the annual limit. But if they were to fund it with their own money, there is very little benefit in it because they typically pay no taxes anyway, and the money won't sit there long enough to grow significantly more because of its tax deferred status than it would in a regular taxable investment account.