In Wireless Sensor Networks (WSNs), each node typically transmits several control and data packets in a contention fashion to the sink. In this work, we mathematically analyze and study three unscheduled transmission schemes for control packets in a cluster-based architecture named Fixed Scheme (FS), Adaptive by Estimation Scheme (AES) and Adaptive by Gamma Scheme (AGS), in order to offer QoS guarantees in terms of system lifetime (related to energy consumption) and reporting delay (related to cluster formation delay). In the literature, different adaptive schemes have been proposed, and also there is research about the appropriate value selection of the transmission probability for the cluster formation. However, it largely overlooked the minimum and maximum values for the transmission probability that entails the best performance. Based on the numerical results, we show that the threshold values are just as important in the system design as the actual value of the transmission probability in adaptive schemes (AES and AGS), to achieve QoS guarantees.