Harmonic and cycloidal gear transmissions are widely used in applications where compact size and high reduction ratios matter, except the two mechanisms rely on quite diverse ways of sharing loads and creating motion. To explore how these differences play out in practice, especially when friction dominates their behaviour, two polyethylene printed prototypes were designed and assessed under identical operating conditions. The study examines both designs during several running cycles at increasing input speeds, first with reduced internal lubrication and later with a light increase of lubrication applied. Temperature and sound were tracked throughout each experiment using a thermal camera and a sound level meter placed at a fixed position relative to the test bench. The results show clear contrasts between the two reducers. The harmonic unit tends to heat up rapidly, while the cycloidal model produces a distinct pulsating noise pattern. Lubrication moderates both effects, but not to the same extent. These observations highlight how mechanical layout and material properties shape the tribological performance of additively manufactured reducers.