Roy I got the Monster Hollow Form Tool from Randy Privett. You can see the tool near the bottom of the page on the webpage.
http://www.monster-wood-tool.com/wst_page5.php
It is a expensive unit (paid $465) but I felt that it is one that I would use and I would have for many years to come. The only thing to wear out would be the cutters. It has a laser so when hollowing you can actually set the depth of cut for the thickness of the walls. It was between the Jamieson rig and this one. For the money you get a extra swan neck so that is why I went this way. I have no experience with the Sorby units.
I do have a set of 5/16" and 5/8" Kelton hollowers for doing freehand. I like them a lot for smaller stuff. Don't know if that helps or not.
By the way my new unit ships tomorrow. I will keep the woodturning forum informed of what I think once setup and I get to use it.
Bernie