For married taxpayers filing a joint return in 2019, at what AGI level does the phase-out limit for contributions to Qualified Tuition Programs (Section 529 plans) start?

There is no phase out limit for contributions to Section 529 plans. The limit on how much money you can contribute to a plan is $15,000 per year, but it doesn't phase out if you have a high income.

The main differences between a Coverdell account and a 529 account is that the Coverdell account has an age limit of 30, an income limit of $220,000 and a maximum annual contribution of $2,000.