A defined-benefit Keogh would allow you to sock away a lot more, with the amount you contribute depending on your age and compensation. It makes the most sense for someone older making a significant amount of money.
So a working couple can contribute a lot more with a Roth 401(k). Each spouse can put $15,000 into a Roth 401(k). It's an opportunity to put a lot of money away that you'll never again have to share with the government.