Math Phd vs Job Offer (struggling very hard in my mind)
These are two very different fields. You have to ask yourself: do you want to be a math teacher or a math researcher? Note that a PhD gears you for research positions. While professors teach, their main duty (at a R1 university) is actually research (this fact is seldom known to undergraduates). Your ability to get a professorship, get tenure, etc. is all based on research. If you are going to go down the PhD track, the choice is for a research-based career. Ask yourself if that's what you want. Choosing a career path based on what you want to do with your life is so much more important than whether a school or department is "prestigious" (which is highly variable depending on the subfield, your adviser, the fellowships/grants you get, etc.).

Caveat: there are positions at 4-year college, i.e. get a PhD and get a position to teach undergraduates.