Experimental characterization of plasma properties along the magnetic nozzle of an electron cyclotron resonance thruster is presented here. A permanent magnet (PM) prototype and a solenoid prototype are tested, whose main difference relies on the magnetic field strength and topology. A cylindrical Langmuir probe is used to measure plasma potential, plasma density and electron temperature. In the PM thruster setup, a laser induced fluorescence diagnostics is performed simultaneously with the Langmuir probe to measure the mean ion kinetic energy, and a Faraday gridded probe to characterize the angular plasma beam. An effective electron cooling rate has been identified, as well as the dependence of the total plasma potential drop with the mass flow rate. Results are compared with a supersonic collisionless fluid-kinetic 1D model where electron dynamics account for magnetic mirror effects and potential barriers, while ions are treated as a fluid cold species. The comparison allows to estimate the sonic transition of the plasma flow.