At my company it seems that most of the people I travel with prefer aisle seats, so what we do is to ask for seats across the aisle from each other or in the next row or something. That way it is possible to say a few words to each other if necessary, but we still have our preferred seating. I also prefer to have an empty seat next to me rather than a coworker. If there has to be someone in the seat next to me however, it doesn't really matter to me if it is a coworker or some stranger (as long as that stranger isn't fat, smelly or otherwize annoying that is ).