Nick F. writes: "Clannad: the Motion Picture from Sentai Filmworks wasn’t the movie I expected — let me back up a moment and say that I’m not really a fan of high school drama, so when it came to Clannad, I decided to approach it as a challenge: how would the movie change my perception of the genre?"
I prefer the anime series over the movie myself for various reasons. The art and story pacing for one. The story is told more slowly and in depth in the anime so you can really savor everything that's happening (mainly After Story).
And I ALWAYS tell my friends to avoid the Clannad movie. "Don't watch it until you finish the two seasons! You'll be completely spoiled."