Here's what I've been advising the LT's in my command and then the ENS/LTJG SNAs...
For the LTs: The 8-10 year mark is really the break even point if you plan on staying in. The math stops working in the BRS favor at approximately 23+ years of service (especially with the cutoff for matching funds at 26) and making O-6. However, BRS could yield more on the day of (actual) retirement (vice working military retirement at age 42). The government, however, will be paying less overall, but it's your call to determine what works for you.
For the ENS/LTJGs: While the government will pay less overall to you as the member, you stand to gain more money overall in the BRS. For the non-prior ENS/LTJGs, I have yet to find a scenario where the member contributes at least 5%, gets the 5% matching, and gets a 5% return through age 65 that has a worse result than the legacy High-3 system.
If you want the excel spreadsheet I am using, PM me. Note that the sheet doesn't take into account withdrawals.
Also, talk to your CFS or go on the JKO or NKO (or MNP now I think is what we are calling it) and find the official calculator (which is still in a beta version, but is ok).