If you do the presentation first, then you won't be thinking about it the whole session and not risk having to interrupt your session to show the powerpoint if you're in the middle of something important.

On the other hand, if you're concerned that the presentation will 'taint' the mood of the session in a way you don't want, then you might wait until the end.

Or if the presentation introduces topics you want to explore in session, then you'd want to do it first.

Bottom line - this is a very personal decision.