NAND is a better gate for design than NOR because at the transistor level the mobility of electrons is normally three times that of holes compared to NOR and thus the NAND is a faster gate.
Additionally, the gate-leakage in NAND structures is much lower. If you consider t_phl and t_plh delays you will find that it is more symmetric in case of NAND ( the delay profile), but for NOR, one delay is much higher than the other(obviously t_plh is higher since the higher resistance pmos's are in series connection which again increases the resistance).
Additionally, the gate-leakage in NAND structures is much lower. If you consider t_phl and t_plh delays you will find that it is more symmetric in case of NAND ( the delay profile), but for NOR, one delay is much higher than the other(obviously t_plh is higher since the higher resistance pmos's are in series connection which again increases the resistance).