If you're in the military, my advice is to max out the ROTH first and then push money into the TSP. Military members' tax rates are artificially low due to the fact that their housing allowance (which can be sizeable depending on location) and BAS are not taxed. So, someone not in uniform that makes the same amount as you actually pays more in taxes...take advantage of being "tax advantaged" with the ROTH. You will likely be in a higher tax bracket later on down the road after you leave the service, at which time the traditional IRA or TSP might be more beneficial to you. Also remember that military members do not get a match for their contributions to the TSP.
Beginning in the first quarter of 2012 there will be a ROTH option available in the TSP. If you're in the military, you'll be deploying at some point (and likely at many points). Any money that you invest while deployed in either a ROTH IRA or the ROTH TSP (beginning in 2012) will never be taxed, and neither will any of the earnings/returns that you get from that money. Considering that when you're deployed you can put up to $48K into the TSP rather than the normal $16.5K (in addition to funding either a traditional or ROTH IRA) could be very beneficial early in your career while you still have a relatively large amount of expendible income, a low income tax rate, and a lot of years to take advantage of the power of compounding interest.