In the private school where I was educated, we were taught that 'nither' was better. No reason was given. A pronuncation is held to be better by certain groups simply as a way of identifying themselves. For instance, snobs say 'tomato sauce', not 'tomato ketchup'. I see no reason for this. Historically, it may be because 'ketchup' was a loan word that arrived much later than 'sauce', but nowadays it makes no sense. I don't think snobbery has a logic behind it.
