I did my homework, read the FAQ on the Wimberly site which is very helpful, got some good responses to my inquiries and figured it out.
For those that might be interested, the difference between the full Wimberly, Kirk Cobra and Sidekick are pretty straightforward:
1) Full Wimberly: Includes panning base, and lens mount is on the bottom. Does NOT require a ball head. Best used with a dedicated tripod. Might be slightly more convenient to mount large lenses on and easier to balance with a flash mounted. Most expensive option, but probably the "cadillac" solution.
2) Kirk Cobra: Includes panning base, and lens mount is on the side. Does NOT require a ball head. Side mount may be trickier to mount large lenses on and harder to balance with flash mounted. Hybrid solution, but almost double the cost of the sidekick since the panning base is included.
3) Wimberly Sidekick: Requires a ball head that includes a panning base. Best option for all but the very largest lenses if you already have a good ball head and don't want to dedicate a tripod to the gimbal rig. Side mount may be trickier to mount large lenses on and harder to balance with flash mounted. May not be best choice for the largest of lenses (600 F4 and larger). Least expensive solution if you already have a good ball head. Definitely the way to go for me.
I emailed Wimberly with my inquiry, and very quickly received a response that included a 10 page document that described the different Wimberly products and compared them with other competitive products like the Kirk Cobra. Very impressive. This document is based on the older Wimberly I gimbal mount, but most of the comments are directly applicable to the soon-to-ship Wimberly II, except that the new model is 1 lb lighter and thus the same weight (effectively) as the Cobra.
Definitely a good reference document. As a convenience to those that might be interested, I've posted a PDF version on my photography site here: http://www.tarafrost.com/download/WimberlyHeads.pdf
Hope this helps others that might find themselves asking the same questions I was.