Honestly, they are both a little lacking for me.

I actually had as much fun on horde mode in Gears of War 2 as I did in Left 4 Dead. I mean, Left 4 Dead just doesn't seem like it will have much replay value outside of custom maps and modding, which will probably be plentiful. But really, killing zombie after zombie gets kind of old. Don't tell me it's a completely different experience every time, it's running through the same places shooting the same guns, but zombies are in different places. At least in Gears 2's horde mode the difficulty is so much more customizable. Me and some friends EASILY rolled through Left 4 Dead on the lower difficulties using only medpacks to melee zombies. I got the most damage received, at about 25. That is way too easy, the expert difficulties were mildly challenging. But the problem is, if you got even halfway decent at them you'd roll through on the hardest difficulty too.

The downside about Gears 2's horde mode is that that's the best addition in the game. The campaign was just meh, the ending sucked, the multiplayer is almost feeling worse. I mean, the shotgun has been weakened, and you don't stun people with it that are running with chainsaws. It's as if running with your chainsaw revved makes you invulnerable. I have hope that they will patch this up, because right now Gears 2 is a complete chainsaw fest and trip mine fest.

Right now I'm thinking Far Cry 2 (PC version). I'd say it will have the best single player. I don't really like Gears MP that much, and WaW will probably fail like Treyarch does. Plus Far Cry 2 also will have a map maker, which is as robust as the separate downloads you need for other games. I have used other level edtiors and they take a lot of time and it's hard to get people playing on them. I spent a ton of time in Forge, and Forge is retarded compared to Far Cry 2's map maker.

I will wait for the reviews before buying any of the games though, I don't like preordering.