Firephoto:

You did not say whether you had other uses for the lens other than your up coming trip. From your original post, it sounds like you are getting new lens for that trip. If you want a new prime and this is as good as a reason as you are going to get to justify the lens (sounds good to me), then the others have more relevent information above. However, if you are laying out this kind of cash for one trip, it may be better to rent a lens for the short time you will be using it. You already have a 24-70 f/2.8L which is pretty nice glass, and it should cover you for any other casual low light shooting you may have.

Pat