This was very close for me. I LOVE both of these. That doesn't mean I can necessarily do either of these for HOURS at a time, but whenever I am in the mood to tackle them I enjoy it.
I gave the very slight edge to Triple Triad. Once you get Wakka's celestial weapon materials there's little reason to play Blitzball any more. Triple Triad helps EVERY character in its game and continues to have respectable rewards per time invested at endgame.