Petit taxis are only for local courses and cannot change city. For instance, you can't take a petit taxi from Rabat to Salé or reverse, it should be a "regular" taxi. Petits taxis are also super cheap and all the ones I took put the counter on without need to ask. This was not the case with the regular taxis, except to/from RBA airport where you have a flat fare at 250 dirhams (about 25 Euros).
I was in Rabat last week and it's really a very pleasant place.