bluti - walls first! Just imagine your lovely, newly sanded and varnished, floors covered in plaster dust, water stains, snots, etc. Better this on the 'old' floor which can then be refurbished after the plasterer has departed ... yes, I know that most trades boast about never leaving a mess, but ...
Sometimes, however, it's not possible to do things in the 'correct' order which would mean laying plastic sheeting (thick stuff) on the new floor. Best avoided!!!