@gloryoftheson
I'm currently doing 15% into the Roth TSP C fund as it is comparable to the S&P 500. I don't get 5% match until 2 years active duty. Should I lower my contribution and max out a Vanguard Roth IRA of $6500. Should I max out the TSP?
I think a good goal for a newly minted O-1 is to max the IRA and get to 50% of the TSP, with the goal of maxing TSP as an O-2. This means investing roughly 25% of your paycheck, and keeping the other 75% for yourself. Note that keeping it for yourself doesn't mean just blowing it on hookers and blow, it means starting up a get out of the military and go to graduate school fund.
Tweak the numbers based on these two financial goals. You don't want to find yourself in 4 years with a 6-figure IRA and not enough liquid savings to make the transition out of the military. But understanding what that transition fund needs to be will allow you to then determine how much more you can devote to your retirement fund.
I will caveat the above with the fact that inflation exists, so a completely valid investment strategy is to max your TSP / IRA the first two years, then dial it down the next two years as you more aggressively save for a transition fund. Which one is "better" will really depend upon what the market does short-term in those first two years.
As far as investing into the IRA: the advantage of the Roth IRA is that you can withdraw your previous contributions penalty free, whereas with TSP you need to take a loan. The other advantage is that it offers a wider array of funds, so you can customize your portfolio with finer granularity than TSP. The advantage of TSP is really in retirement when you have access to the G fund - this is essentially the social security fund, and if the net assessed value goes down the federal government pays back the difference.
Write a lump-sum check if you can. Many of the lower cost fee funds require a $3,000 initial investment. Once you get over 15k into the IRA is when you can really start to tweak your portfolio, until then you're going to be stuck in 1-2 funds tops.
Roth is a good choice for you considering your early aggressive investment plan, which would incur tax penalties later on as your income
increases from the compound growth.