i think stock would be the better way to go when your just starting out. because it helps you know exactly what the car needs.
does it understeer? does it oversteer? does it need a 1,1.5,2 way lsd? youll find out when you drift it stock. some cars dont need alot, others need some work. it also challenges you to overcome obstacles(IE low power, no lsd, soft suspension, hard suspension, long turns, tight turns, increasing/decreasing radius turns, etc.) that you normally wont come across with a tuned car. this all helps develop your skill. sure the cars are more fun the faster they are, but you really need to start out stock to see what a car needs, if anything. then when you add parts, you can say hmm.. this car understeered in stock form, ill add a bit of toe out on the rear to help even it out. or hey this car oversteered alot stock, ill add some negative camber in the rear to help give it a litte more grip. or hey this cars braking balance is off.. ill adjust it using the brake balance adjuster. this car seems underpowered, ill add a bit to help it get around basic corners.
as you can see, learning to drive stock cars before modified cars can be really helpful on learning ways to get around problems you may encounter on a specific corner, and also helps you learn how each adjustment made to the car does to affect it.