Start by looking at your income. There are income limits for Roth IRAs, so if your income is above those limits, then it's a no-brainer: a traditional IRA is the only one for you.
If you are eligible for both a Roth and a traditional IRA, then you've got to run some numbers.
In general, a traditional deductible IRA is appropriate if you expect to be in a lower income tax bracket when you retire. By deducting your contributions now, you lower your current tax bill. When you retire and start withdrawing money, you'll be in a lower tax bracket, giving less money overall to the tax man.
If you expect to be in the same or higher tax bracket when you retire, you may instead want to consider contributing to a Roth IRA, which allows you to pay your taxes now.
But it can be difficult, if not impossible, to guess what tax bracket you will be in later in life - particularly if you've got a long way to go until you retire. So consider spreading your bets. For example, if you already have a tax-deferred 401(k) plan through your employer, you might want to invest in a Roth IRA if you are eligible. 
