Being a poor Giants fan living in So. Cal, I don't get to watch many Giants games. So stuff like ESPN Gamecast and MLB Gameday are very useful for me. While both systems have their advantages, it's rather difficult to use both at the same time.
Gamecast seems to have the more polished interface. I can actually use all of its features and everything shows up the way it's supposed to. However, on Gameday, I often have problems especially when looking back at what happened in earlier innings. The scrollbars are sometimes to short to allow me to look at what happened in the entire inning. And I have yet to see the mysterious lineup tab. It's always blank, (and I don't think it's a feature for paying customers). (If anyone knows how to fix this on Firefox 3 on Linux, I'd love to know.) Gamecast also features slightly better hit visualizations, i.e., it's easier to see where someone hit the ball, and I get to see hitter's hot and cold zones for free.
But Gameday's killer feature that keeps bringing me back is it's pitch identification. It's nice to know whether Schierholtz is striking out slider's at his back foot, or checking how much break Zito has on his curveball. And of course it's great to bitch at the occasional "strike" on Gameday that was obviously a ball.