Two different operating system types are currently considered for sensor networks: event driven and multi-threaded. This paper compares the two well-known operating systems TinyOS (event driven) and MANTIS (multi-threaded) regarding their memory usage, power consumption and processing capabilities. TinyOS and MANTIS are both ported to the DSYS25 sensor platform. Both operating systems are used to execute the same sensor network application and the aforementioned parameters of interest are measured. The results presented in this paper show for which set of applications each operating system is preferable.