I played a Series II soprano for about 4 years and just recently got a new Series III. In my experience, the III is brighter, has better intonation, far superior ergonomics, and responds better in the high range. Altissimo G on the II was resistant, but it pops out on the III. I can get up to a C, whereas I could get to an A on the II. It also sounds to me as though it blends better in quartets or other ensembles. Overall a better horn.
As for the weight thing, I play it with a strap but it just hangs loose. I usually play sitting (quartets) and I rest the bell on my knee. Problem solved
