Simply for your consideration, the SL6 has a better drivetrain and better wheels. I see the retail price difference is about $900 but I bet a good LBS could whittle that down to the out the door price difference of $500 or less.
Oh, and to your original question, the new Domane is rapidly gaining popularity as a road bike/gravel bike hybrid for those that like to do both.
Last edited by BengalCat; 02-10-20 at 03:50 PM.
Reason: clarity