Some of us may have differing opinions about the ending songs of the two games. I think they are both equal in quality, maybe the second one being more of a broad song and the first being specific to Portal. I think the surge in popularity in the first one was due to the fact that such an ending song was not done before. There is usually a random pointless song or the main theme in an epic form at the end. Yet Portal was odd in that up until the end there was no "music" until the credits.
Also, it hasn't been mentioned, but please don't spoil the game for me. I will return to my Xbox in about 2 weeks, and I will definitely get it the first day I come back.