There is basically one major benefit to an IRA vs. a 401(k) (really two sides of a coin). With an IRA, you can pick any mutual fund, stock, ETF, bond, or a number of other investments to put your money in. A 401(k) is limited to the dozen or so mutual funds that your employer has selected. In the typical 401(k) plan, there is a significant fee for the plan and the choices are all high-expense-funds. This arrangement is good for the company administering the plan, and bad for the participants.
That's a general rule however. My company offers a (relatively) awesome 401(k) with a bunch of low-expense funds, a token annual fee, and even allows me to invest half the money directly in individual stocks, so you should definitely check out the details and compare.