Leave which type of asset to children, Roth or taxable brokerage?

I would like to leave some money to children. I have pretax 401k and taxable investment account. My question is, after retirement, should I use my taxable investment to pay for living expenses and do some Roth conversion? Or should I just use my pretax 401k to pay for living expenses and leave the taxable investment to kids with the step up basis?