Hi Francesca,
If I understand correctly you plan to provide access circuits by means of gigabit interfaces only (no serial). As you wrote the urban centers collecting many (not one) access circuits with GE interface. In this case obviously the right choice would be using switch. The model choice is greatly depends on other things like (throughput, qos, routing protocol etc.). If you use purely whatever Ethernet interfaces then L3 switch is pretty good choice, because L3 switches have much better performance compared to routers. SO you have the choice to migrate the whole network based on switches or you can use them as let's say media converters to connect the access circuits to the routers.
Krisztian