Neither.
Although i do like Killzone to a degree, i never finished the second game. (And i'm occasionally playing the first on my PS2 when i'm in the computer room, as my PS2 is hooked to a small TV beside my monitor) and i don't feel there's any point in getting the third game before i've completed the second. Multiplayer isn't all that important to me and the main game can't be that much different from its predecessor, except for advancing the story and one or two quirks, like the jetpack section and some minor refinements. It's not worth dropping £40 just yet.
I'm not purchasing Bulletstorm, as i felt the demo was somewhat lackluster. It didn't really feel like an FPS, it felt like a Tony Hawk game gone a bit weird. I don't like the whole "score points for crazy kills" system. I find it a huge pain in the arse to repeatedly come up with elaborate ways to kill things. Sure it's novel the first couple of times, but then you realise the whole game is built around this mechanic, and it just feels tired and forced. I didn't like the grapple or the slow-mo effect from said grapple or kicking someone. I tried to be 'badass' and kick a guy down a hole, but the slow-mo effect had him ever so slowly falling to his demise before it wore off, that i wasn't sure if he'd fall down the hole and actually die at all. It just felt incredibly awkward. I prefer a fast paced, no holds barred shoot 'em up. I was looking for something like the original Painkiller title, i guess. I didn't get that. I got some kind of weird Gears of War meets Tony Hawk hybrid. Some of the discomfort might also be down to it using the Unreal engine, as i associate first person shooter games using that engine with Unreal Tournament as i used to play it extensively, and it's a far cry from that. I just didn't like it.
So instead my £40 has gone to a myriad of budget titles and second hand games that i would have otherwise overlooked. And i couldn't be happier.