I think Waheguru as a word is quite important apart from the breath control it synchronises, it also makes you use the bone structure in your face to extend inwards and outwards almost taking the full range of facial exercises. As an exclusive name for God, there is none and for sikhs there should be no question about this subject. As has been mentioned before devotion is what matters. I have found it easier to do simran of Waheguru as it flows one after the other, Ik Onkar however feels like I'm starting again each time I say it, sound's weird but it's what works best for me. There have been arguments also that the specific parts of your palate that are touched during the recitation of gurbani has a positive effect on your general wellbeing. As for a historical imperative for the word Waheguru there is none. I think when sikhism first started everything was rich in spirituality and that thick (as in thickness or richness) climate generated a gurmantar and the word Waheguru became pargat. Alot of people refer to accounts of various sources which breakdown the syllables of the word and some say it comes from the salutation of the Khalsa but I think we have a habit of ignoring the conditions prevalent from a spiritual side of view. The effulgence of spirit was greater in those people in spite of the historical pressures on them. There devotion was unwavering, this devotion we can pick up on when we examine different records from history in different mediums also, we are lucky as other religions don't have access to peer into our past like we do. We have music recordings, photographic evidence, literature, art, architecture all preserved which when delving into shows what a religion is really like when it first starts and it is this devotion that we learn is what needs recapturing. This is the only way we can understand what we are and how we need to go. Some will argue that repeating a word is not anything to do with practicing SIKHI but I would argue that the true act of worship involves identifying your target with a name and then repeating it with love.
