Which is better for a graphic design job, contract-base or as an employee?

First job, employee. You will find that you have MOST of the chops for doing it when you finish your education. You want to do two things: make money of course but also build your portfolio. So go with him for whatever you can afford and di the best work you can. Once you have some chops you are going to have to move on anyhow but the first experience of a workspace and a steady supply of projects provided by someone else will be invaluable.