Just to give you an idea of the difference in investing tax free money vs. Taxed. If your wife has any earnings at all, you will be solidly in the 22% tax bracket in the current year. Let's assume about an 8 percent return investing somewhat aggressively in the stock market, which at your age you should be.
So $20,00 tax free can all be invested now and in 21 years will have about a 500% return, so you will have $120,000 which will be taxed when you withdraw it. Assume you will be in the next lower tax bracket at 12% and you will pocket $105,600 during your retirement years.
If you invest after tax dollars like the ROTH, you will only have $15,600 to invest as you sent $4,400 to uncle Sam. That money will grow tax free and again about a 500% return. You will have about $93,600 in your account of which $15,600 you can withdraw tax free. The remaining $78,000 will be taxed at 12% leaving about $68,640. So in total you will have $84,240 that goes into your pocket during the retirement years.
So in the end, there is more than a $21,000 benefit for your retirement years to investing pretaxed dollars vs. after taxed dollars.