I have been thinking about this for a while. I don't want to contribute anything to my 401(k) above what it takes to get a full employer match, so I wonder where I should put the excess. Here's my take:
Traditional IRA
Benefits:
Tax deductible now. If you have income being taxed at a higher tax bracket, throw it in your IRA to reduce your tax burden.
Disadvantages:
Cannot be withdrawn early unless you're willing to pay taxes and a 10% penalty.
This is the route to take further into your career if you think your income in retirement will be reduced. Maybe you will have your house paid off, etc. and right now you're at your peak earning years.
Roth IRA
Benefits:
Interest earned is tax free.
Can be withdrawn without penalty at any time (I'm sure there are limitations)
Disadvantages:
Must contribute post-tax dollars.
If you're at lower tax brackets right now, or you've put all your higher tax bracket earnings away in a traditional IRA already and you still have some money to save, this is the place. Also, you can use this as a pseudo-emergency fund, since if you need the money, you can withdraw your original contributions without additional taxes or penalties at any time. You cannot withdraw your earnings from a Roth until retirement, though.
So both accounts have their merits. You would need to tell us more about your current situation for a better idea of which is right for you.