Postdoc vs Assistant Prof - which is better for a career academic?
In the US, I think it would be clear that the assistant professorship is preferable to a post doc in nearly every case. The exception would be the opportunity to work with a superstar as a post doc. 

Again, in the US, assistant professorships are (almost) invariably tenure track, though there are a very few colleges that don't offer tenure at all. But that isn't likely to be a factor. 

For most, a post doc is really just considered to be a stepping stone to a real position and a way to continue to advance if the job market is poor at the time. 

But ask the institution itself about tenure and also about the likelihood of actually earning it.