Is it just a single choice at the end of the game?
Then the answer is obvious! Choose foe!
What would make a MUCH better game is if they had to make the decision many times during the game. That way there is an incentive to choose "friend" in the hope that the other player will reciprocate.
Check out the book
The Evolution of Co-operation by Robert Axelrod. It is essentially a mathematical treatment of this game, and what is the best strategy to adopt. Very interesting, and has many diverse real-world implications (economics, the origins of society, international reations, etc).