I simply chose my field based on what I knew I enjoyed. I already had a job with working in systems, and I had HPC/FPGA experience from undergrad. Basically, I choose topics that I had at least a basic understanding and desired to expand into mastery.

I will be honest and say I don't think you've really thought this through. I would really weigh the pros and cons of the degree. Think about what you'd do instead of pursuing a degree. Identify a specific job you would want after you get your degree. If you only want to teach, you could be an instructor at a University, but the pay will not be good. If you want to do research, doing reaearch at a university level will require a PhD. Research centers will hire MS holders, but as engineers rather than researchers. If you want an industry job, make sure you actually need the degree you are thinking of getting.

For 99% of people, carrying on with the degree they have is the better choice. In your case I'd either make sure you consider the above or only pursue a master's if you truly cannot get the job you want with self-teaching.