I am considering either 13F or 13B. Which is a better MOS?

13F Hands down. Lots more options.

13 F. Why? Forward Observers ate able to be assign to Field Artillery units. But FOs can be assigned to FA units and infantry units like 75th Ranger Regiment. I promote the Rangers because I'm a product of them. You get to experience the best of both worlds.

I would not say that any MOS is better than the other.
Every MOS has an impact and a role for the military, there is no better or worse.
However, all I can say is that whatever fits you better will suit you.
I suggest read about each mos about what their focuses are on, look up them up on youtube as well.
Ask questions per to each soldiers who are on that MOS and their experiences with what you can expect from it.