Yes, the Roth TSP will act like a Roth IRA. The advantage of the Roth IRA is it allows diversification into some assets not available in the TSP such as emerging market funds, real estate investment trusts, and inflation protected bonds. So, if you don't need those asset classes and the TSP funds meet your needs, I don't see a problem with contributing to the Roth TSP first. Fill your Roth tax advantaged space first, especially at our low tax rates.
There really is no gouge on which TSP fund is better. They each do different things. There is associated risk vs return for each fund. Except for the G-Fund...it's a free lunch. There is no other asset out there these days that will give 3-4% return and protect your principle. I'd recommend reading into asset allocation plans. There are tons of ways to skin the cat.
If you are even thinking about this stuff, you are ahead of 75% of the general population.