ESA- $2000 contribution, income limit
529- no income limit, basically no contribution limit (some states prevent additional contributions if the account is worth $350,000, but nothing stops you from opening additional accounts with other states), some state tax benefits, some special estate tax benefits, usually high expense ratios and limited investment options
So generally I'd lean towards a 529. Especially if you're trying to sock away more than $2k a year (which will pay for about a week's worth of classes at a private school these days!)