With the squad they've taken, they simply have to use either Bomber (it's much shorter to write this than other variants of David Hussey... may have defeated the purpose with this comment...) or White. There is no proper replacement for Hogg, so they're going to want as many slow options as possible, especially with Symonds looking a bit ginger after the 3rd Test.
Watson and Clarke are definitely the most qualified for opening. Watson racked up a spate of good scores in those tournaments in India and Malaysia while he was opening, generally at a run a ball or better once he got going. Clarke might average better down the order, but opening is where he gets to make hundreds. His style of aggression isn't suited to slogging out the death, but it is very suited to tight opening fields.