You've really got to run the numbers for yourself and the odds of you staying in to 20. Obviously if you make it to 20, the legacy system is much better, because you get the "free" pension that starts the day you leave the service. And you can do your own TSP investing on top of it. With BRS, you don't get those TSP dollars until after you retire (If I recall correctly).
I don't know if once the GWOT starts dwindling we're going to enter into a new cold war with China, but we may start seeing a draw down of forces like we saw after the fall of the USSR. If that happens, people who thought they'd be in for 20 might start getting pushed out/incentivized out.
I'm also O3 with a long way to go till 20, in a job that could see big cuts if combat operations really slow down. So I'm still not sure on what I'll do myself. Right now I'm leaning towards staying with legacy, basically as a high risk high reward gamble, as I'm currently single and able to save a lot on my own.
ETA: Remember, you've got all of 2018 to decide as well.
And here's some more info here from a previous discussion.
https://texags.com/forums/63/topics/2862873/replies