Firstly, what is wrong with your air suspension/ if anything?
I have springs on the back as standard on my Disco 2, and mine rides great, I don't tow anything heavy and I dont have any electrics/compressors to go wrong!! I have never experienced the ride quality with SLS on the road so can't compare...
On the other hand I know people who would not get rid of the rear SLS ....so it's down to personal oppinion/preference.
Do you use the Disco off road? Remember you wont be able to raise/lower it for a little extra clearence if you do...