I researched this long and hard before I bought and in the end went for the SSL and fitted it myself.
To get the optimum set up on the "Rutherford" setup or similar fixed spring methods you have to make sure you have exactly the correct length springs to get the correct interaction between the main and rebound just kissing the stub axle whereas with the SSL you can adjust it to get that relationship perfectly setup. Even then there will be some settlement in the first few (500 - 1000) miles and you can readjust with the SSL to get back to the correct setup.
You can't readjust the fixed spring setup without fitting new springs or shimming it and that means stripping it all down again or going back to the dealer.
So adjustability is one big benefit but don't underestimate the effect of the secondary rate riser spring on the SSL. It means you can start with a slightly softer main spring which give a very compliant cruising ride but than stiffens up with more aggressive cornering and absorbs some of the pothole bumps as the rate riser spring comes into play.
And a final benefit for me is that the preload is adjustable so when you are fitting it you don't have to compress all the preload. You can fit it with very little preload and then adjust to suit when everything is in place and fully contained. IMO much easier and safer. ( Although compressing springs is not necessarily unsafe if you do it properly and take the right precautions.)
As a generalisation, you can buy and self fit the SSL for about the same price as a dealer fitted "Rutherford" setup.
But then you also get the satisfaction of having sorted your own front end whilst really understanding how it works and being able to "play" with it in the future.
BTW, when I did fit the SSL it was a huge improvement over the standard setup both in cruising and spirited driving.
I fully support everything Bob says. I fitted my SSL setup on April 12th 2013. I needed to adjust it a couple of times in the first 500 miles but since then I've not needed to touch it, it's been perfect.
Wild horses on their bended knees wouldn't persuade me to go back to a stock setup.