A5/1 is the stream cipher algorithm used in GSM communication in order to provide over-the-air communication privacy. The objective of this paper is to compare the security levels of two (2) multiplexer-based A5 crypto systems. In this research also, we improved our previous model which is more complicated with five (5) linear feedback shift registers (LFSRs). So, less complex models were proposed by adding a multiplexer which is able to produce the random key generator. Several statistical tests such as frequency test, serial test, runs test, long runs of ones test and linear complexity test were used to test the strength of the algorithm.