Post-doc vs. Assistant Professor choice, but neither ideal I'll keep this question short to make it easy to read, but happy to supply details later if relevant.

I recently finished my Ph.D. - computer science, in the US. I have a choice between accepting a post-doc position and a tenure-track Assistant Professor (AP) position.

However:


The AP position is at a satellite campus of a state university. This is a masters-granting campus (i.e. no PhD students). The teaching load is 3+3, though there is expectation of decent research output, especially (but not exclusively) during the summers.
The post-doc is at an R1 research university (though not highly ranked or especially prestigious). It appears (to me) to be less-than-ideal for the following reason: the PI applies techniques from my field (CS, AI) to another field (let's call it F2). He publishes (and presumably, I would publish) exclusively in conferences and journals of F2. After 2-years of this, I fear being stuck between two fields, departments of neither seriously considering me for tenure-track positions.


So, both positions appear sub-optimal, which is the source of my dilemma. Eventually, I hope for a tenure-track position at a R1 university. Which of these will best help me meet this goal?

ADDITION:

Thanks for the responses. It's helpful that the responses clearly categorize the AP institution as a teaching-focussed university. When I went to interview, they harped on their focus on research, show-casing the research activities and publications of professors and MS students. Since it is a masters-granting university and not an undergraduate teaching college, I was uncertain of what to make of all this. I suppose, in the end, the 3+3 load is telling.

One reason I'm swayed toward the AP is that, as a foreign national, a tenure-track position is a firm step toward US permanent residency, which a post-doc position is not.
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.