The great coach Mark Rippetoe is the to go source for strength training [1]
This (unofficial, but true-to-source) site is a better resource for the actual Starting Strength programs if you don't want to buy the book: http://startingstrength.wikia.com/wiki/FAQ:The_Program
Why would that be true? I believe the only difference is that SS recommends 3 work sets and SL recommends 5 sets, and SS uses power cleans while SL uses rows. Do the additional sets work better for people starting from low fitness levels?
Yes, because they cannot handle too much weight and so, their training volume is very low even on 5 work sets. Note that the body will catch up pretty quick and 5x5 will be too demanding after 9-12 of solid training.
Also, SS is much more technical because of the power clean, which is a very demanding exercise. You cannot possibly do 5 sets of power cleans without putting you in risk of injury.