Dear hps,
You may choose the one that is more comfortable to you.
"Waheguru" as Gurmantar is given to us by our Gurus. It is simple to pronounce. If you look into the previous posts, you will get some guidelines.
"OM" is an Ancient Mantar. For it to be effective you need to take a personal lesson from someone who is familiar with the vocal. Incorrect pronounciation will yeild no results.
Make sure it is done with "Bhavna" feelings.
Good luck