Kerry M. Kerstetter made an interesting point at his Tax Guru-Ker$tetter Letter site by saying "[d]eferring current tax deductions - or worse still, paying actual taxes on IRA conversions - based on the promise of free pay-outs decades down the road is too much of a gamble for me to feel safe with." His theory: although the current law spells out that any gains in Roth IRA accounts are free of tax, people in DC can redefine the law when it fits their political motives. He even brings a good example: social security tax is not deductible based on the "promise of completely tax free benefits later on" -- now that up to 85% of social security income may be subject to tax if the recipient falls into certain income category.
Fortunately I don't need to decide between 401(k) and Roth IRA now. I can always max out my 401(k) contribution (and my wife's) as well as putting $3,000/year for each of our Roth IRA accounts. Kerry's post made me a bit uncomfortable, but I don't think people in DC will be as aggresive as to tax the original Roth IRA contributions (which is already after-tax money), so I cannot be worse off by moving some money in after-tax accounts to Roth IRA accounts.