So here are some numbers to ponder.
You are currently in the 25% bracket. Meaning that contributing to a roth will require you to pay 31.27% of the dollars you put into it vs 0% in the 401k.
With your top end guess on retirement savings (750,000) you will be looking at about $30,000 per year of income from the savings. The top end of the 15% tax bracket for married couples is $74,900. We also need to take into account your personal and standard deductions worth $20,600. Meaning that you can have $95,500 in retirement income and still fall in the 15% bracket. The maximum social security income is $31,956 this year. Meaning that at most your income would clock in at $77,934 per year ($30k savings + Max SS + spouse SS benefit). This leaves $17,566 of room for other income sources you may have and you would still wind up in the 15% bracket. And this is assuming you qualify for the max social security benefit.
Unless there is a hefty pension or other stream of income you haven't mentioned, I think the 401K is the obvious choice.