Correlation of upper critical field (Hc2) and critical current density (Jc) with processing temperature of nano-C doped MgB2 has been studied in comparison to SiC and pure MgB2. SiC and C doped MgB2 exhibit opposite trends in the dependence of Jc and Hc2 on sintering temperature. This is explained by different reactivities of carbon available upon creation of MgB2 for the two types of doping. Nanocarbon doped MgB2 requires sintering temperatures in excess of 900 °C to obtain high boron substitution for carbon, enhancing the vortex pinning and impurity scattering of charge carriers. However, carbon substitution in nano-SiC doped MgB2 occurs at less than 650 °C, allowing lower sintering temperature and high degree of carbon substitution. Both pure and SiC doped MgB2 benefit from low sintering temperature, which results in more grain boundary defects. Substantial carbon substitution can compensate for the disadvantage of sintering at high temperature of nano-C doped MgB2, giving the best Jc of 4.8×103 A/cm2 at 4.5 K and 12 T. This is comparable to the low-temperature sintered nano-SiC doped MgB2 wires.