Amsterdam is very tourist. Too many people and too much hype.

I grew up in Holland Rotterdam, and I use to go to SENSI (Nieuwe Binnenweg), but it got shut down because they got caught selling to kids under the age of 18.

Rotterdam is much more quiet, relaxing. You don't have a few hundred people all crowding into these cafes to catch a glimpse of it. Much more mellow. Try some of these:

-De Poel (Eendrachtsweg 29)
4 floors and many activities. In the center
-De Lachende Paus (Nieuwe Binnenweg 345b)
a famous coffeeshop in town to buy but it has no place to sit down. Just take away. Great choise of pre-rolled joints. In the center as well.
-T Trefpunt (Botersloot 7)
with free internet. In the center too.
-Sensi smile (Henegouwerlaan 73)
very popular.
-Reefer (Oppert 1)
beautiful interior.
-Out of Time (Nieuwe Binnenweg 363)
-Pluto (Nieuwe Binnenweg 54)
just to buy things, no places to sit down. Great supplies of smoking accessories.

Hope that helps you somewhat.