Of course the Falcon can be compared to the SLS. Both are vehicles designed to take things into space for crying out loud!
The figures are easy to compare as well. Just include these factors in your comparison! Even if we don't know exactly what those factors are, we can make a ballpark estimate the total difference and add that as a percentage to spaceX's final figure.
The idea that things can't be compared because they are different is nonsense. When things are different is the only time comparisons are meaningful to make!