Thomas Domingues I recommend security. The reason being, it'll be around and has stood the test of time for thousands of years, the job of protecting people and their assets is extremely fulfilling both technically and ethically, and it also involves some of the most technical and advanced knowledge and problem solving in the industry.
See, you asked AI, Big Data, or Security, but what you don't realize is that working in security, you could use AI to help secure big data at a company. So potentially, you could use all 3. But again, you must have a goal and security is a great goal and facilitator. You also asked for a story and some personal experience so I'll explain how it works out for me:
I work for a computer antivirus company as a malware researcher and it involves software engineering, reverse engineering, operating systems knowledge, and even cryptology. The neat thing about programming is that it can be used to solve many problems using computers in general, so say all I ever did was full-time reverse-engineer. I would still be a software engineer too because I would be writing programs which help me to reverse engineer. However, I end up working with SQL to analyze data, Python for tooling, C, C#/.NET, JavaScript, and more. It's a lot of fun and to be honest with you the work never stops or lets up.
Now, let's talk about the other options. You could be a penetration tester/ethical hacker as it's called, a network security specialist, exploit researcher, threat analyst, security engineer, and more. The big question is, how often do you want to be looking at and working with code? If that's a big thing to do, stress your coding experience and look for an engineer job like reverse engineer or security engineer or researcher. If you don't mind a little less code/programming, try pentesting/ethical hacking, which is basically where you are scanning for vulnerabilities and security holes and reporting them. What it comes down to is, are you a security minded person? Do you enjoy thinking like a bad guy would? Do you pay attention to those types of details? Are you the kind of guy who always checks that the doors and windows are locked and is aware of his surroundings? If so, security may be for you.
One of my favorite things about security is that you have to be the best, period. You have to go above and beyond and understand the entire system inside out. The reason why is because the bad guys do and if you don't, you won't last long in security. "It just builds" is not a thing in security. You must understand the underlying implementation and how it all works to understand the flaws and vulnerabilities.