Hmm... that seems like asking, "Blonde or brunette - which is better?" Who cares? They're both great!
But seriously, I do think Fable and Halo2 are both great games. Like FableFreak, I like them about the same. I just recently bought my Xbox and I bought these two games to go with it. I find myself switching back and forth between them depending on the circumstances. Sometimes I feel like relaxing, exploring, questing, and fighting in the world of Fable. Other times I'm in the mood for more action and feel like splattering the walls with alien innards.
Also, it depends on how much time I have for a gaming session. I don't usually play Fable unless I have a few hours to kill, since I like to play at a leisurely pace, exploring all the nooks and crannies and doing the side quests. If you're more of a power-gamer and you like to plow through, you might be disappointed with the relative ease and short duration of the main Fable storyline (there are plenty of posts about that all over the 'net). If I only have a short time to play, Halo2 fits the bill. However, I just completed Halo2 the other day so I guess I'll be primarily in Fable mode for a while.
The other factor is multiplayer mode. I haven't gotten into multiplayer Halo2 yet, but by most accounts the multiplayer is awesome!
In summary, I give 'em both two thumbs up. If I HAD to choose, I guess I'd give the edge to Halo2, but only because I'm an old FPS fan since the days of original Doom for PC. Not to mention that the reason I bought my Xbox was because I really loved Halo for PC and could not resist the call of Halo2.
