First, there's not really a business form called a "co-ownership" even though the FAA allows that to be listed on the registration. In general, co-owners are effectively "partners" in either a general or limited partnership. "Co-ownership" as most of us think about it is actually a general partnership, and general partners personally responsible for all debts of the owned entity, while owners of an LLC have only their investment in the LLC at risk for debts of the owned entity. In addition, any general partner can commit the entire partnership to agreements.
Of course, the LLC won't protect the owners of the LLC from liability for their own actions (i.e., flying the plane, making maintenance decisions, etc), but it does mean that if one of the other LLC owners prangs the plane and someone is injured, and the first LLC owner is not found to have negligently allowed/caused the accident to happen, the first LLC owner's risk is limited to what s/he has in the LLC, and other personal assets are not at risk. In addition, unless the LLC conducts its business like a business (documents signed by company officers, no mingling of personal and company funds, decisions made as a company and documented, etc), the LLC's protection can be pierced and the personal assets of all owners attacked. However, if the owners of the LLC play by the rules, there are substantial protections provided by the LLC form of business entity.
So, what do you think is the best choice to limit one's legal liability?