Reference letter, personal vs prestigious?
A letter should highlight your skills, which usually only come out when working closely with somebody. Getting a letter from a renowned professor saying that you were sitting in his lecture is no more useful than listing this course on your CV. Hence, go for the "personal" (I guess it is work-oriented, rather than saying "Woofas is a nice chap who always entertains at our pub quiz night") letter, and highlight the course with the awesome professor elsewhere.