Originally Posted by elmo_2006
Actually, the 50mm f/1.2L is not part of the 'holy trinity' of primes, maybe merely a disciple (the 'holy trinity' usually refers to the 35L, 85L, and 135L).
What is the planned venue for this shoot, and how much working room will you have? What is the separation from the background you'll want to blur out? If you have a little bit of working distance, the 85L is an amazing lens, but if there's decent subject-background separation, the 70-200 (assuming I recall correctly that you have the f/2.8 IS II) will do well, and give you a lot of flexibility for framing.
If you really need the lens only for this session, have you considered renting?