You are asking the wrong question. We assume you are putting enough into your 401k to get the full employer match. That is critical. But beyond that, adding to the 401k may or may not be a good idea.
1) Are you saving enough for retirement for your needs? If the answer is yes, then you don't need to put any more in and stop reading this thread. Although, likely the answer is no.
2) Ok, you aren't saving enough for retirement if you reached this point. Now the question is do you put the money in the 401k or do you open a Roth IRA? The 401k isn't taxed now, the Roth IRA is taxed now. The 401k is limited in investment choices and likely to have higher fees. The Roth IRA is unlimited to investment choices and is likely to have lower fees.
3) Do you need the additional choices from the Roth IRA to get diversified investments? If so, take the Roth IRA. Are your 401k fees dramatically higher? If so, take the Roth IRA. We can't answer these questions for you since we don't know your situation.
4) What is left is the question about deferring taxes. Don't let people with bad math (especially the so-called experts) fool you. If your tax rate now is the same as the tax rate when you retire, there is no difference. People will try to tell you otherwise without doing the full math (or they'll pretend to do the math in a way that sounds good), and they will be wrong. But what if your tax rate now is higher than your tax rate when you retire? Well, in that case, you want to pay taxes when you retire - put it in the 401k or traditional IRA if you can. What if your tax rate now is lower than your tax rate when you retire? In that case, the choice is easy, pay the taxes now when they are low - put it in a Roth IRA.
5) Of course, you won't know for sure what the tax rate will be when you retire. But if your income is low now, and if you think the social security problem/medicare problem/Democrats being elected will raise taxes in the future, you can have a good idea of the answer. Taxes are historically quite low now. You are probably best off taking advantage of it and paying taxes now.