not that scary, but it has a really dark atmosphere that really sucks you in
Dark Sector's atmosphere basically consists of you going "Oh shit my dude is infected with a disease! I can throw around this little twirly blade thingy and lop people's heads off! Awesome! Look at that blood! Hell yes! I just sliced that dude's leg off! Woo!"
The story is okay, I guess. Main character is double-crossed by government. Main character's best friend who has the same infection as him is the bad guy. Oh no, tragic plot twist. Blah blah. Nothing you haven't seen before. The game play is fun though. Throwing the blade and then controlling it in mid-air like Xena: Warrior Princess is freakin sweet. I really HATED fighting anything other than regular humans, though. Stupid infected monsters sucked. I think I finished the game in about 6 or so hours? I'm not sure because I played it over 2 days and didn't really keep track of time, although I didn't play it that much because I've had papers due the past week. It's a great rental but I really wouldn't recommend buying it.
I'd even go so far as to say that Army of Two has more replayability than Dark Sector. At least with Army of Two you have multiple guns and upgrades and co-op. Plus the levels in Dark Sector aren't really memorable and they all feel the same. Being able to use different elements with your throwing blade-thingy is pretty neat but feels a bit gimmicky. You're better off just using a power throw and aiming for the head. Even with the guns in Dark Sector, you don't really feel like you're accomplishing anything. You're better off to use the throwing blade thingy than to shoot someone.
I can't really comment on which game has the better versus multiplayer because to me they're both pretty crappy. In Army of Two you at least have objectives and everything is fairly balanced. Dark Sector's versus is absolute garbage because of the cheap ways you can kill someone. "OH HAY I'M BEHIND YOU PRESSIN B AND KILLIN YOU IN ONE BLOW LOLOLOL!" Whatever, though. It's like trying to figure out if shit with corn or shit with peas is more nutritious for you, either way you're eating shit.