It's better infamy wise, but as you're gonna get cores on all of Germany you don't need to do anything else besides it, so there's no problem with infamy.
The annexing missions come after all the other generic missions, about 50 years later, at which time you already got a core on magdeburg.
So, that would be my strategy, maybe you can vassalize one or two other provinces required for this, but having a lot of vassals is really not the best idea, it takes longer than directly annexing.
Btw, try to choose a nation whose's sliders are easy to get to free trade, that way you gain money and that way you can invest it in Land technology so that you don't have to worry about Bohemia or whoever the emperor is
That's what I do
Note: This suggestions are to form Germany quickly, you could also make a super empire and form Germany whenever the option comes.