I own both games, if I had to recommend one of them I would pick Flower. While both games don't have a whole lot to do, Flower is the only one that makes you feel like you are doing something, moving towards a goal.
No matter which one you choose, you will be doing the same things over and over again, just to get to a new map to do the same things over and over again. Flower however is artistically stunning and the music will move you.
I hope this helps a little.