CLA Propagate Term: Xor vs Or Gate
For computing the carry propagate term both the expressions 


Pi = Ai or Bi
Pi = Ai xor Bi


works since the only difference in the truth tables of XOR and OR is when A=1 and B=1. However in the following expression:

C1 = g0 + C0(p0)

the P0 becomes irrelevant when Ai=1 and Bi=1 since the Carry Generate term (AiBi=1). For this reason, in case of a CLA adder the XOR and OR gates are logically equivalent.

The OR gate is better than XOR gate for large circuits since the implementation takes lesser number of CMOS transistors. In VLSI, usually gate count is a critical factor for Area and Power considerations.