It probably doesn't matter much, as neither state allows tax deduction of contributions. Go for whichever has better investment options. The accounts I've opened are with Vanguard for the low fees, which is technically a Nevada account. The money will spend the same later.
Also, the person who opens the account controls it, including the possibility of reassignment of funds. Assuming you are contributing, you probably want to do so to an account you control. Grandparents may want the same, but multiple accounts can exist for one beneficiary. You just need to trust the beneficiary's SSN to the person opening the account. If you don't trust the grandparents not to steal your children's identities, insist they contribute to an account you control.