If you keep the ownership in joint tenancy, if one couple dies, their interest would go to the other couples, not their own family.  In an LLC, the "business" owns the property, and each couple has their ownership interest in the LLC which is personal property, not real estate, and would go into their estate at death.  Death would force the LLC to be dissolved, and either a new entity would have to be created or the property would have to be sold or divided up.

Nobody is going to be able to work out what the best solution is to this, without sitting down with the six of you and getting into more detail about other assets and potential liabilities, among other things.