Helicon plasma sources currently represent an active field of research in the domain of low-temperature plasmas due to several interesting characteristics for in-space propulsion applications. This work reports direct comparison of krypton and xenon plasma properties through spatially resolved measurements performed in the near-field plume of a subkilowatt-class 13.56 MHz Helicon source. The set of developed and employed intrusive diagnostics is comprehensively described. The magnetic components of the plasma rf field are inferred using a 3-D B-dot probe along the reactor axial direction with and without the externally applied DC magnetic field. Plasma floating potential fluctuations are measured using a capacitive probe to draw design requirements for a rf-compensated Langmuir probe. The latter is used to perform axial and radial measurements of plasma density, electron temperature and plasma potential. A 4-grids Retarding Potential Analyzer is used to infer the ion kinetic energy downstream the source exit. The effect of magnetic field magnitude on ions and electrons properties is especially investigated.