The main functional difference between the two bikes is the Trek frames grade of carbon. That's it. (SLR has a choice of colors, SL7 has just black.)
The SLR has a Trek's 700 grade carbon that is slightly lighter and stronger. The SL has 500 grade carbon. All the other components are the same, wheels, drivetrain, etc.
The difference in weight is the SLR is 1/2 pound lighter Why would anyone want to pay $1800 more just for choices of color and a 1/2 pound lighter bike?