Hi I am not a window cleaner but will be. So please don't expect my advice to be tried and tested, I just though one reply would be better than none.
So...I would personally hire, simple really. More profit. If you earn £40 an hour, you would fairly pay subbies 50%, if you hired you could make them happy with £10 an hour therefor you earning 75%...
There is many problems involved, right staff, decent finish, reliability etc.
But same with subbies but more, they could find there own work and jack you off, mainly less profit, surely thats why your in this game?
So... more profit may be more work, maybe less either way YOU benefit. Good luck