I bought the MFC-6890CDW back in November. Had the MFC-J6510DW been released yet? If so, I didn't see it. I mainly saw the MFC-6490CW and MFC-5890CN as alternatives, which have no support for duplex printing.
Brother printers is catching up with the speedy development of printer's features. And I believe that is actually what it should do since the market is a raging world. HP now had its partnership with Google and so on.