Sorry fellas, I gotta claim ignorance on most of the questions. If I'd had a hoe with thumb to compare to, I could tell you the difference. I did not even get a chance to look at this stuff before hand, I told the salesman what I wanted and he took it from there (a pretty straight up guy for a salesman, and I generally don't trust them.) I did get to look at a 7500 Woods before hand, and picked the 9000 based on the performance and size comparison. The cost of the 9000 was about 1000 bucks more. Glad I upgraded. It seems to be well matched to the 4610. The pump gives decent performance at low engine RPM, I like running at 15-1800 RPM rather than the 540 PTO speed of 2700 RPM or so, that would drive me nuts.
As far as the thumb goes, I'm sure there are better (and more expensive) choices. This one was 450. bucks installed. Like most of the stuff I've seen from Rankin, they use fairly heavy steel and iron, simple design and finish (not as nice as some other equipment but better than most import stuff I've seen)
How would it work for rocks? I will find out for sure later this summer, I can tell you it can pick up small stumps with a single bound... It would probobly pay to use care, since the curl force on the 9000 is probobly more than the thumb wants.
The pins would likely serve as a shear safety, and it might be a good idea to wrap the thumb tines with rubber for a better grip, maybe a bicycle tire? I got a chance to reinstall the hoe yesterday, first time with not a clue, just shy of 1 hour. As Andy mentioned, it can be done in alot less once you get the hang of it. I included the time it took to go get the manual, scratch my head, "what the ....." and so on. It really was not bad, but I would not care to do it on uneven ground.