With the maturity of the cloud infrastructure, many data are moved to the cloud. However, availability and security of the cloud data still remain the major concerns. To relieve these concerns, it has been proposed to disperse encoded data redundantly across multiple independent cloud providers, so that at least a certain number of data fragments are required for data recovery, thus called cloud-of-clouds (CoC) systems. As for system performance, there is always a trade-off among sufficient security or high availability with different settings on the number of fragments and the threshold value. Here in this paper we extend the scenario and propose a comprehensive hybrid model that provides a holistic framework taking into account the diverse reliability and security characteristics of the storage providers in a CoC system to enhance the enforcement of data protection. The availability and security of the system in our model is analyzed theoretically in a systematic way. Simulation results also show that with our model, by exploiting the asymmetrical characters of the CoC system, both security and availability can be achieved satisfactorily.