Update:
The SSL12 works well with the new MBP. 
I think the Apogee Duet with my old 2010 MBP is a bit warmer, more forgiving, easier to match volume levels. 
The SSL12 is clear, records well, a bit more exact(?), direct sounding. There's definitely a learning curve there with the new GarageBand software and new interface. 
The SSL 12 has more intuitive software, capabilities of more inputs, in addition to the 4 on board.  Lots of cool free software to explore.
All in all, so far, the SSL12 seems like it's a very capable and great sounding interface.  I look forward to growing into its full capabilities, when/if I ever get around to hunkering down and getting serious with recording.  
Happy to answer any questions.