Preference of NAND & NOR gates
NAND and NOR are preferred because they are smaller and use less power in a CMOS process than equivalent AND or OR gates. NAND and NOR gates can be created with 4 transistors, while AND/OR require 6.

An AND/OR gate is laid out in a cell library generally as a NAND/NOR followed by an inverter.



AND Gate (OR is similar)