Rehearse or memorise to improve my presentations? This may be a dumb question. I would like to improve my presentation skills. 

So should I rehearse my presentation at least 5-10-20 times? Or should I memorise my presentation and practice it enough?

When rehearsing, I make my presentation in front of a mirror and observe that I deliver the same messages in a different way in each time, while if I memorise what to say for each slide of my presentation, then I deliver the same messages in same way very confidently each time I practice in front of a mirror. 

So I am getting confused which one is the better method to deliver an effective presentation?
It's not "either rehearse or memorize". Also, your sentence "In rehearse, every time I make presentations in front of mirror and observe that I am delivering same messages in a different way in each time." got me thinking that you probably rehearse inefficiently.

The purpose of rehearsal is to improve your presentation. So after every rehearsal you should reflect about your performance, think about points that did not come across right, and about ways to improve the presentation. If you just start over again without reflection and without a new, improved plan, you do a weird mix of "not really rehearsing" and "not really memorizing".

I'd say, a good rehearsal includes quite some memorization for the crucial parts. However, if you rehearse long enough, you may get to some point where do not even memorize your lines, but where the presentation has a natural flow such that your previously memorized phrases come out naturally. 

I'd say, bare memorization is ok, but may be dangerous if you don't know the story line of your presentation as a whole. Imagine that you loose you lines and get stuck in the middle of the presentation. So also rehearse to speak freely but memorize some lines that you find particularly good at crucial points. A good structure of the presentation and a good story is very helpful here (and for a badly structured presentation, neither memorizing nor rehearsal will lead to a good presentation, anyway).