If you're going to be a researcher, pick the one you care most about. You'll need the extra motivation and drive to stay passionate for the next 40+ years.
If you're going to be a professor, it really doesn't matter much which you pick, as they are both quite viable, and both have interesting sub-fields to focus on.
If you want just about anything else, pick the crypto. The demand is higher for practical comp sci, leading to better job stability in recession years. With the ever-increasing need for better encryption and foiling enemy systems, both the private and government sectors will continue to see a rise in demand (and pay) for any smart cryptographer.
Note that depending on individual university programs, computer science degrees may include a lot of emphasis on topics outside of cryptography, or may assume a working knowledge of certain comp sci topics. For example, if you don't have a background in programming, it would behoove you to chat up the professors and make sure they don't expect you to already be conversant in C++, Java, object oriented concepts, etc. With your masters I fully expect you could learn whatever you lack, but that's a choice you want to make and not have thrust on you after getting into a program.