Shaft drive and belt drive has no big different if setup is done properly. However they of course are different but what I would like to say is the differency is quite little now.
Basically belt drive gives smoother acceleration and deceleration which is easier in handling. While shaft drive gives more instance response in compare with belt drive which make the car more agressive. But in fact there is a lot of other factors affecting the response of a car such as internal gear ratio, electronic device's combination, and etc. Someone believe that shaft drive is more efficient than belt drive. But in face they are so close because of improved technology in nowadays.
People used to choose belt drive for mod class and use shaft drive to handle low power motor (23t) was because they would like to have the advantage of higher drive train efficieny in shaft drive car and more instance response. For mod class, because of the high power output, a smooth acceleration/deceleration become more important inside the race. But it does not mean we cannot use shaft drive to run mod class. The advantages/disadvantages are still there. It all depends on whether you can handle it or not. Someone has used a TB Evo 5 to enter the A main in IB cup and always stay fast all over the 2007.
Torque steer and gear strip are also the mentioned factors which make people think that shaft drive is not suitable to run mod class. Again technology improved, degree of problem is decrease a lot. remember my LCG services me very well even using plastic gear under a 7T output!!
Shaft drive is an orange and belt drive is an apple. However they both keep you health!