Mike,
there are really only two options for a sidecar; solid or completely free, also called a leaner. Once the sidecar is solid, the whole steering geometry becomes like a car; in a leaner application, the bike steers and rides like a solo. Anything in between would be a muddle, and probably very difficult to handle.
For an off-road application, rigid is best. Higher ground clearance and a soft suspension, no different from a solo bike. The suspension can be helped with a sway bar, linking the rear swing arm to the sidecar swing arm. For a road rig, a leaner is an option, and one that I just may have to do for my next build; just because it is so unique.
http://www.kalich.de/index.html(the link is German, but Kalich is a leader in that field, and has a few good videos)