I don't know anything about national postdocs in the humanities, but gosh, I would go with the SLAC job. In my field, the postdoc is what you do in order to get the job... which you've already been offered!
I think I have to agree with menotti. It doesn't sound like a very difficult decision. As one half of a dual-body situation, I find your TT job possibility quite enviable:
You will not have to move.
You will not have to separate from your partner.
You will have a TT job.
Your partner will continue to have a job.
You will be in better financial shape than if you take the postdoc.
You can try to have children while living in the same place.
So often we have been trained to think in terms of "big names," whether those names happen to be names of universities or fellowships. Because of this, it almost seems like we are giving up or if we don't automatically salivate when offered something by a big name of any kind.
The problem in this case is that there is no guarantee that the big name postdoc will land you a TT job in the same area as your partner in two years. You don't need a postdoc to put you in that position; you already have an offer. Gift horses, mouths, etc.
Good luck. I wish my partner and I could be as lucky as you!