I don't disagree that you're likely to find quality Professors at law schools of every rank, but what's different at higher-ranked schools is typically the student body quality and the level of discussion and understanding that potentially creates. There's also economic signaling.
While that Professor who teaches at multiple schools may use similarly difficult exams, that doesn't mean that his teaching style or the in-class experience is the same. Supposedly, higher-ranked schools teach theory more and lower-ranked schools teach the black letter law. At a higher-ranked school, with an objectively higher student quality, it's more likely the Professor doesn't have to go over the basics but can talk about nuances or theory of the law because it's assumed the students can learn it on their own. Students in class at other schools may need more black letter law training, which could help them more with the bar exam.
I guess the point is there's no right or wrong answer to this question because it's whatever you think will serve your needs best. Just my two cents.