Junior faculty position - course development
I think this is a false dichotomy. In the US, you are generally expected to be able to teach a course your first semester. You shouldn't need to have the course fully prepared in order to start teaching it, though. You can prepare as you go. 

Also, many departments in STEM fields back off on their standard teaching load for junior faculty during the first year to give you some time to prepare your first courses, so you might get a 1-1 your first year instead of the usual 2-1 (or whatever). This kind of thing varies quite a lot from department to department, but there is usually some sort of consideration given to let you ramp up some classes.