Gyani ji,
I don't fit the bill, and I've never given this any thought before. The solution would be to do some research, but this I'm not good at doing. But being the more knowledgeable member in this regard, I'll express some thoughts.
First, I think that you are wrong to make the kind of comparison. Sikhs unlike Buddhists do not have the monastic tradition as part of their system of thought. In Buddhism there are monks, nuns (for want of a better word), laymen and laywomen, whereas Sikhs I think, only have laymen and laywomen. So if you want to compare, then it must be between laypeople and not laypeople vs. monks.
Comparing laypeople we can see that while Sikhs encourage keeping hair for all, in Buddhism there are no rules in this regard. A bald Sikh is one who would have gone against the tradition, whereas a bald lay Buddhist is just one of many possible choices and he is no more or less a Buddhist by virtue of this. Monks on the other hand must all have a shaven head. Why, I am not sure because it has never occurred to me to try to find out, but I do have some thoughts and will give them to you.
A monk, idealistically, is one who has given up the household life after seeing the dustiness of such a life. He therefore has decided to lead a simple life without any possessions except the basic requisites needed to sustain life. Hair as we know is object of vanity and it is my guess that one of the reason why monks shave off their hair completely is to remind them about this. Besides if monks were allowed to keep their hair, how long or how short, with sideburns or not, mustache and beard or none, these would be considerations that clearly take the attention away from the real aim, namely the development of wisdom and detachment.
Also in a community, there have to be set of rules both in conduct as well as appearance so as to allow smooth functioning of the whole. If therefore, monks were allowed to keep hair, there'd be so much distraction for each individual followed by comparison and this is chaotic. And I suppose also, that no hair is practical in terms of hygiene and this is in line with the spirit of a simple life.
But I'll try to find out more when next I meet my friends.
I think you could have avoided making the kind of remark as in the last one!
Anyway, the first monks under the Buddha were all enlightened whose behavior was perfect and therefore no rules existed at the time. Gradually as time passed by, people with less than perfect understanding and virtue started to ordain and one by one conducted themselves in such a way that rules had to be created so as to discourage repetition of such behavior. These rules were however all in line with natural laws such that any monk with understanding will see them as reminders about what is good and worthwhile and all of them supports for the development of understanding.
But of course as time passed by even more, those same rules became such that monks would either follow or break them but with no understanding either way. And this is the situation of Viharas in KL and everywhere else in the world today. So I don't think that these are the places one goes to in order to ask the kind of questions with the expectation of getting the correct answer.
