score:4
the train system is inefficient and often late. my recommendation is the take another form of transportation called louage. you can take it from Tunis (monsef bey station -- "محطة سيارات ألجرة لواج "المنصف باي--according to this source) to Tozeur for 27.3TND according to this source (تونس - توزر 27300)
you can buy the ticket from the booth in the station (they don't take resevations)
monastir and sousse, both have great beaches and a lot of places to go. In terms of resorts and hotels, they both have amazing options. for monastir they have a beach resort area that is slightly secluded from the city if you want to enjoy some quiet time with less crowded beaches. the commute to the city isn't very long and the city, in general, is less expensive. as for sousse, you can enjoy the closeness to the city and the convenient transportation within the city but if you are closer to the touristic area, the beaches will be more crowded (the city is generally more expensive). sousse can open for you the option to travel through ennfidha airport though which you can consider if you haven't booked yet.