I suggest you should choose a noble and I'll tell you why. The "king" and the archbishop are fighting together for power (in the beginning), you don't know wich one will be the strongest, that depends at the nobles. As a noble you are free to choose the side you want, like I allready said a king (only if you start to be the king) is unsure about his future, maybe you are the first one who's out of the game. But a noble can change side and make his own desicions, with this you will probably survive much longer, and if you play well you can allways claim the trown later