They are both great dogs, but completely different. I have both.
Labs are definitely the playful, affectionate puppy like dog. GSDs are more reserved, especially around other, but can be very affectionate and have their playful side.
Both require a ton of exercise, for me, my lab requires less. A field bred Lab will tend to need more.
That means more than just a walk around the block. Labs are smart but GSDs are far more intelligent. GSDs often need a job. Remember, they are a herding breed, bred to spend long days running and using their mind to control the herd.
Both can become very destructive and may develop behavior issues if left with no outlet for their energy.
I would not be looking for a dog to guard. Many many people misunderstand behavior dogs are exhibiting and call it "protection". And without proper training, there is no guarantee the dog will actually protect when push comes to shove. The size and sound of the bark will be more than enough deterrant.
Personally, my favorite breed is the GSD. I do love the Labs too (mine is my heart dog). I love the look of GSDs, the way they carry themselve, their intelligence. I don't use the word protective, but my GSDs are always watching what is going on.
You also need to consider the amount of commitment either of these breed require. You are young, and will be busy. Remember that while you are in class all day, your dog is home alone. Very often there is alot of social activities that go along with university. there are still times I stay home because I have worked all day and it is not fair to them to be alone again. Its one thing to leave them to go to school or work, but if you are out more than you are home, its not fair to the dog. They are pack animals and crave companionship.
Plus consider what to do if you go on a trip.
Either way you can't go wrong. Just make sure you have the time to train and exercise whatever you get.
ETA- an GSDs are not stubborn. They are very trainable (hello, they are trained to be Police dogs, detection dogs, etc), but can sometimes be challenging to someone who is not sure of what they are doing.
owned by 2 gsds and a lab