First of sorry for the upside pics, I tried to flip them and everything, but no dice...sorry.
We'll my wonderful wife just surprised me with a rotisserie for birthday last month, and I've got to say I love it. I guess it's a new one because I find that it fits well. It's got a little lip that slips into the kettle and mine is tight (see close up picture).
It doesn't really work on the WSM to well though. It will sit onto the body fine, but the lid of the WSM has to just rest on the top of the ring. It would work, you would just have to be careful not to bump the lid and knock it off.
I plan on doing some chickens this weekend, so I'll get some better pics when I've got more light. I think the weber one would fit a turkey, you would have to probably take out the grate, and the motor is supposed to be able to carry 20 pounds.
Which ever way you go I'm sure you'll love it!