What does the Starfarer have to offer over the similarly sized Hull C? Why would you want to buy a Starfarer specificaly over the other space trucks?

Fuel is the name of the game here. While the Hull C can store fuel pods (and the Starfarer can store cargo pods,) the Starfarer is further designed to do something with them. The hull superstructure has the 'plumbing' for actually using the fuel in those tanks, rather than just storing them.