Graphics: When I was playing multiplayer Halo 2 at my friend's place, I wasn't overly impressed with the graphics. Especially when I compared them to GoW. So I was a bit afraid that my pre-ordered Halo 3 would disappoint me on this aspect. I was wrong. The graphics in Halo 3 are excellent (I'm watching on an HD TV). Everything is very sharp, there's a lot of detail and some really cool effects (like the explosions).
Still, the game is less realistic than Gears of War. This is because of the more cartoon-like characters and the usage of brighter colors like purple and green for vehicles and weapons. In Gears, everything is more gray. This doesn't make the Halo graphics worse, just less realistic. A designer's choice.
Sound: The sound is just great and I'm just listening to my TV's built in sound system. I can imagine a surround system really makes the game come to live.
Also the supporting music helps create a mood supportive of the mother of all fights. At times the music even brings some rock 'n roll into game especially when most of the fighting is done from atop an armed vehicle.
That said, it's clear that the GoW story still needs to develop in future episodes which will need to explain why human race can't understand yet why the locust can't back down.
The Halo 3 story on the other hand is clearly coming together in this episode when at some points an explanation is given for some events in Halo 2.
I'm not sure the Gears of War team cares as much about the story as the Halo team. In GoW, the dialogs need to support the mission at hand. In Halo 3, the dialogs and clips often paint the bigger picture about why there's this fight.
How does it play? This is a tough one. Gears of War is definitely the more linear type of game with narrower play fields and a more limited set of options. For instance in those phases where you need to be in a vehicle, the game will put you in a vehicle. In Halo there's a lot more freedom and flexibility.
The fighting is also vastly different. While you really can storm ahead in Halo 3 and shoot and melee your enemies, GoW invites you to take cover, snipe, sneak up… Also in Gears of War it seems you have finer control over your aiming. I love Gears for that.
The weaponry in Gears is way more limited than Halo's but feels more real and each weapon clearly has a different style and feel. You get some of that with Halo but, to me, the difference is not as clear.
Now don't get me wrong, I truly enjoy playing Halo and the options I have sometimes truly surprise me and keep me learning things even while I'm already further in the game.
Also fundamental is whether you prefer a third person shooter or a first person shooter. Is it that hard to let the player choose? When offered the choice, I would certainly opt for 3rd person but I must say it bothers me less than I expected.
My conclusion: Everybody is raving about the game and I'm one of the fans. The game really pulls you in and you want to fight until the end. Only more story and more flexibility will allow Gears of War 2 detrone Halo 3.
I wonder how long it will take me to finish the game. Only after I finished the game on Heroic I think I'll dare to go play multiplayer online.