why we generally prefer nand logic over nor,though both are universal gates??

well, it depends on your requirements from the circuit.

This difference in performance between NAND and NOR arises because of CMOS architecture.

In NAND, the NMOS transistors are arranged in a stack and PMOS transistors are connected in parallel.

In NOR, the PMOS transistors are arranged in a stack and NMOS transistors are connected in parallel.

In any circuit, if PMOS is used, it results in larger propagation delay. That's why NMOS devices are generally preferred.

But there are cases too where NOR gates are preferred