If you use a 529 account and use the money for college, then you won't be taxed on the gains in the account (and you might get a state tax break).
On the other hand, if you use a 529 account and don't use the money for college (and you don't fall in an exception, like a scholarship), you will owe tax on your earnings at ordinary income rates (not capital gains) and you will also owe a 10% penalty on your earnings.
Obviously it's up to you which way to go, but I wouldn't put more in a 529 than you expect to use.
There may be other ways you can save that will reduce taxes, such as a Roth IRA, or buying mutual funds/ETFs, will at least make you eligible for capital gains treatment.