That is a question that will vary depending on your personal financial information. For most, the Roth would probably be the best vehicle, because of the lower fee structure and the tax-free distributions available after age 59 and a half. However, there are situations in which the 403(b) is also an appropriate vehicle. That is a situation that you should talk to a personal financial advisor so that you can review your overall goals and financial situation and then make an informed decision. It also depends on what funds are available in each. If there are better-suited fund choices in the 403(b) portfolio, then perhaps maxing out the 401-k to the employer's matching portion is good, then investing the rest into the 403(b) for a more suitable return.

Many questions are fairly easy to answer here, but this one requires too much personal information to be given over the internet. Not good.