What's the benefit of delaying a tenure-track position for doing a postdoc (when you already have a tenure-track offer)?
Once you have a t-t position, you're in a race against time to get as many publications as you can before your tenure clock runs out.

A postdoc effectively gives you one more year of focused attention to research and publications before you have to start teaching. 

Note that teaching can be highly disruptive to research and publications, especially in your first year.