Zekkyou on 29 June 2014
Having played both I'd go with Portal 2. The puzzles are well designed and fun, the story is interesting, there is a near endless supply of player created content and it has some of the best writing you will ever find in a video game.
As a bonus Portal 2 is incredibly easy to run but still looks visually appealing. Yay for source, the mother of all potato game engines.
That all said, you can't go wrong with either. Both are great games.
