Rotterdam > Amsterdam
Having visited Amsterdam for a few days before coming to Rotterdam, I much preferred my time in Rotterdam, which is cheaper, far less seedy/touristy and in general a more pleasant experience. Whilst the architecture is very different to that of traditional Amsterdam, the cityscape is still very impressive with high rise glass buildings everywhere and open boulevards and squares and whilst it doesn't have the same level of things to visit as Amsterdam, there is plenty to see and do.
Insider Tip:If I was to make a recommendation, I would say split your visit between Amsterdam and Rotterdam - go hard for three days in Amsterdam, see the sights, smoke the weed etc. and then catch the train to Rotterdam (1 hour from Amsterdam) and gain a much more authentic Dutch experience.