Post-doc vs. Assistant Professor choice, but neither ideal
It is uncommon for people to move up from a teaching school to an R1 school. It happens, but rarely, and your chances are slim. So this seems like a dead end regarding your goal, though of course teaching can be quite satisfying for a lot of us.

The offer of a postdoc elsewhere at least allows you to continue dreaming about a permanent position at an R1 university. Of course, there is never a guarantee that you will get such a position, whereas the AP offer you have at least offers this certainty.