What looks worse on a resume - gap after graduation, or unrelated certificate?

Which looks worse?

The gap.

A gap looks like you have something to hide and chances are something will start poking at it in the interview anyway. It's better to tackle it upfront and put it out there. That saves you from the question "why did you not put this into your resume?"

There is nothing wrong with: "tried it, got the T-shirt, didn't like it and moved on" unless what you are applying to is heavy on Field B in which case you probably shouldn't apply there anyway.

Gaps are red alerts for recruiters, so always avoid gaps or they'll assume the worst.

Any form of studies is a legitimate reason for doing something else for a year, so there's no need to hide that part even if it's completely unrelated to your current profession. It is common that people try out several things before settling for something.

Additionally, what is going to matter the most on your resume right now is your current, relevant work experience in Field A. That's more important than both degrees. So if this was your first job in the field after graduation, put emphasis on that job and mention details of what you've been working with.