I am co-owner and partner of a small group but moonlight for a large subspecialty group. I would never want to work for a large subspecialty group. Regardless of your status (owner, partner, xyz), they tend to treat you like an employee/cost center and you deal more with administrators than you care for. You are paying their salary – but usually they do not behave like it. Not every private group is democratic but at least you know after a while what the decision structure is. It is very difficult to be fair in a multi-spec group but as a subspec doc you seem to often have the shorter stick and subsidize more areas of the practice than others.
In both cases I would try to talk to former docs and get as much outside information about both groups. In my own search, I also was never given full access to all financials but enough and also got enough information from partners who had left about the financial and decision structure that I felt I made a solid decision. Your post sounds like you need still more information.