The main difference between the two American actuarial societies is their educational/credentialing paths.  The CAS has one fellowship track for property and casualty, while the SOA has fellowship tracks for enterprise risk management, finance and investments, life insurance and annuities, retirement benefits, health insurance, and general insurance (aka property & casualty).  An actuary's job and employer determine which society they'll get their credentials with.  While both societies educate property & casualty actuaries, currently most property & casualty actuaries practicing in the United States get their fellowship with the CAS.