GTC is easy to get into and achieve your goals in. It is quite short though.
GT3 has some parts that are more difficult eg some of the license tests and some races.
In GT3 you can buy stuff to upgrade your car whereas in GTC you can't.
In GT3 you can adjust many settings on your car whereas in GTC you can only change basic settings.
GT3 is a lot longer than GTC. In GTC any race you win or license you get adds about 2% to your game completion whereas in GT3 it is more like 0.1%.
In GTC when you get to 100% you have unlocked all the cars you can. In GT3 there may still be more cars that you want.
If you think you will end up getting both then I would start with GTC.
If you are only going to get one I would get GT3.