The human brain is set up to act on cues. If you memorise your presentation word for word then each word becomes a cue for the next. If you forget one word then the cue disappears and you risk drying up.
By all means write your speech out in full and memorise some key phrases. Focus on memorising the ideas in your presentation and how they connect to each other.
Know your structure, know your subject, know your ideas and the words will form themselves.