One scenario: Put it in a 401k if you are...

1) close to retirement age

2) in a high taxeable income bracket, say 28%.

Suppose make a lot of money and put in a a Roth IRA. You would have gain 28% on your investment before you break even had you put it in a 401k. With the markets the way they are these days, this might take many years. If you are going to retire soon this might never happen.

So anyway, a lot depends on how much you make, when, and how long before you intend to retire.

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You don't need a weatherman to know which way the wind blows - Robert A. Zimmerman