Like everyone else said, it's purely preferential. Due to the recent sales of the two, I've explained this quite a bit to a number of different people, and will do the same here.

Gameplay type: Fluid, Dynamic

Left 4 Dead 2 is a more co-op centred game, with good teamwork needed to survive till the end. It is generally geared more towards casual play, though the difficulty can be pretty tough on expert. You don't really need to know much about the mechanics/ maps/ weapons to do adequately at it, and can be really fun to play in a 4v4 inhouse versus. The campaigns are generally very fluid and the mechanics well polished, and the AI director is a nice touch. The voice acting and music cues are also excellent, and quips from the characters do make the game seem a little more natural and fleshed out. The gameplay is generally more dynamic in nature, and game experience can vary very greatly every time you play it.

Gameplay type: Static, Mechanical

Killing Floor, on the other hand, seems to be a more old-school, hardcore game. There really isn't any story other than killing waves of zeds, and can get a little repetitive. On the easier difficulties, teamwork isn't really as important, as there technically isn't a zed that can "pin" you down, though it requires much more teamwork than L4D2 as the difficulty can ramps up. The difficulties cranks all the way to murderous for this one, and the game compensates by including a 'perk' system, which is basically just a class system, where you level up by killing stuff, doing damage etc. This gives it a sort of grinding aspect, which drives it away from the casual floor, but helps to keep the game fresh, due to the number of roles you can play. Because of the intense difficulty, knowledge of the mechanics is essential in doing higher difficulty games, as the error margins becomes small enough that a slip-up can cause a team-wipe. There isn't much in the way of voice acting in this one, and music is mostly minimal, what little the characters say add mostly to the 'camp' value of the game. The gameplay is generally more mechanical in nature - you require a more calm, focused and trained mind, as the later difficulties heavily bias the game against you.

In conclusion, Left 4 Dead 2 is a fantastic game - a very fluid and polished 'movie', while Killing Floor is more of the gritty B movie that shows it's flaws, but is what really gets the adrenaline moving with it's more oppressive atmosphere and "ohshit" moments.

P.S. Everything mentioned above is purely my opinion, after having played hundreds of hours of both. YMMV :P

l4d without doubt

I've tried both. I only tried L4D2 yesterday, but already I think it's better than Killing Floor.

Both equal to me. Like someone said, Killing Floor has more guns, where as in l4D, you only get like 3-5? But, as again, like someone said, you can play as the zombies in L4D, so yah, both equal games to me.

L4D2 :)

Both

Left 4 Floor

l like both

I like both games but I love L4D2 more. It has different campaigns and has versus, survival, mutation and scavenge mode. Although Killing floor has creepier Maps and Zombies!!! But yeah, the game play is quite repetitive.

Left for Dead 2 :)

Got to say Killing Floor, I like how people can create their own maps and there are tons of weapons, more customization too.
On the other hand, if L4D2 is getting Steam workshop, it will also have user made content, I also like that it allows you to move up at your own pace, instead of having waves.

for that price, both!

L4D2 is all around better if you don't want to spend half the game running to the store and making math equations in the forced downtime.

i just buy them buth :)

Killing Floor better :D

I finish the campaign of left4dead2 like 2 or 3 times then i got bored, meanwhile i have played around 700hours already played on killing floor and i never get tired since there is lot of content coming out on events and stuff. L4D2 its more polished but its too much linear, killing floor has a outdated engine but gives more freedom to the gameplay.

Killing Floor cos it doesnt get boring for me like L4D2 does cos I played through each campaign a few times and lost interest but Kf didnt start boring me also the seasonal events make it a lot more fun cos l4D2 doesnt have seasonals , just my personal opinion on it so dont take it personally .

If I had to pick one, it'd be L4D2, only because the gameplay is slightly more unique than KF.
KF leans more towards team-deathmatch-vs-bots (it's not really that simple, but it leans that way) where L4D2 is scenario based, forcing you to try to make it to the next checkpoint. Both have waves of attacks and special Zs, both are good, and both have an initial oh-crap-I'm-dead learning curve and a longer-term skill learning curve. Also, as others mentioned, you can play either side in L4D2.
Imho, the community for KF is better though. Initially, you'll be votekicked out of Versus L4D2 maps all the time, until you learn how to play.

Killing Floor = CoD Zombie Mode

I like Killing Floor much better. It's slower-paced. You kill single enemies rather than just cutting through an undefinable mass of undead. It feels more tactical and less repetitive to me. Levelling also gives you a sense of progress. You will have accomplished something, even if you die. And the summer/halloween/christmas events are fun.
L4D2 has way nicer visuals, of course.

Killing Florr

Both L4D games are boring compared to Killing Floor. Get KF for sure.

Played both a lot. If you are only looking for Coop maybe KF is better. But the Versus mode of L4D2 is just awesome and way better than KF imo.

Many thanks to you all, guys! So many responses/comments.
I got great overview from both games, so thanks again.
I decided to buy L4D2, but as I can see that many of you recommend KF, I have to buy it too but then when I get bored of L4D2. Then I think I have money to buy KF bundle (christmas(Y)).