We compare the performance of two different mid-infrared nonlinear crystals, type I and type II AgGaS (AGS), and type I CdSiP (CSP), in a synchronously pumped optical parametric oscillator (SPOPO) pumped at 1064 nm. CSP showed a clear performance advantage at 6.5 μm idler wavelength in comparison to AGS. The oscillation threshold with CSP was three and five times lower compared to AGS type II and type I, respectively. The highest quantum conversion efficiency was obtained for CSP. It allowed us to reach 27.5 mW of idler average power, compared to 13 and 6 mW with type II and type I AGS, respectively. However, the tuning range was narrower with CSP than with AGS in both type I and type II configurations. The performance of the CSP SPOPO makes it suitable for generation of high power picosecond pulses at 6.45 μm for surgical applications.