I wrote a brilliant guest post at Don't Mess With Taxes, titled Roth IRAs and Your Retirement Income. (Note - this article now reflects 2012 rates. Just updated)
Simply put, it's an ongoing question of whether your taxes will be higher now than at any point in the future. If you are in the 25% bracket now, it would take quite of bit of money for your withdrawals to put you in that bracket at retirement. In the case of the IRA, you have the opportunity to convert in any year between now and retirement if your rate that year drops for whatever reason.
The simplest case is if you are now in the 25% bracket. I say go pre-tax, and track, year by year what your withdrawal would be if you retired today.
At 15%, but with a good chance for promotion to the 25% bracket, start with Roth flavor and then as you hit 25%, use a combination. This approach would smooth your marginal rate to stay at 15%.
To give you a start to this puzzle, in 2012, a couple has a $11,900 standard deduction along with 2 exemptions of $3800 each. This means the first $19,500 in an IRA comes out tax free at retirement. If you believe in a 4% withdrawal rate, you need a retirement account containing $500K pretax to generate this much money. This tick up with inflation, 2 years ago, it was $18,700 and $467K respectively. This is why those who scream "taxes will go up" may be correct, but do you really believe the standard deduction and exemptions will go away?
Edit - and as time passes, and I learn more, new info comes to my attention. The above thoughts not withstanding, there's an issue of taxation of Social Security benefits. This creates a The Phantom Tax Rate Zone which I recently wrote about. A single person with not really too high an income gets thrust into the 46% bracket. Not a typo, 46.25% to be exact.