I have both and my eyes really can't tell the difference. Both have great glass and unless you are putting them thru side by side tests, you probably won't notice any difference. The SIII has a 30 mm tube which will give you more adjustment of travel but it will weigh more. The only reason I would choose one over the other would be that one does not offer the choices that I am looking for or I can get a better deal on one vs the other.