Killzone 2.

What has Ensemble developed before?

I looked it up.  They made Age of Empires and Age of Mythology series.  But have little to no experience on consoles.  Usually their games finish runner-up to Blizzard's games for PC Strategy Game  of the Year Award.  They'll probably win Console Strategy Game of the Year this Year unless Blizzard makes a multi-plat version of StarCraft II.

Age of Empire series

I would normally go with the Halo franchise but since this game is not a FPS, I could see Killzone 2 outselling it, especially with all the hype and good reviews.

Halo brand or not, Halo Wars is a RTS therefore it has no chance of outselling Killzone 2. I can't remeber one single rts on consoles that has even sold over a million. Besides Killzone 2 is almost like the Halo for PS3 right now, and if it's even only half as good as they say it's going to sell atleast 3-4 million total.

Halo Wars will sale more, really because of the brand name

Halo Wars will sell more　due to recognition. KZ2 despite the hype does suffer from a bad rep from past games.

Short answer: KZ2 easily

Long answer:

Don't use Halo wars as a comparison as it's a RTS and till now no RTS has managed to sell even a million on a console...KZ2 is a shooter and needs to be compared with other shooters.
To me in its 1st year KZ2 has to sell at least as Gears of War (first one)...both are (or were in the case of GeoW) ultra hyped game and PS3 can now count on a 20M userbase (vs 6M X360 at the time of GeoW's launch).

This thread is full of damage control: for KZ2 anything under 4M in its first year  would be somewhat disappointing given the hype surrounding this game (big shooters any given year manage to achieve this result and you can't consider KZ2 just like "another shooter")...on the other hand anything over 2M (1st year) would be simply exceptional for a RTS no matter the name it brings (that would be more than double than the best selling RTS in the history of console gaming)

@ Spectrumglr:
I know that is must sound crazy to compare a FPS with a RTS. But if there is any RTS which has the potential to sell over a million on a console then it's Halo Wars. I think it's possible that HW can be for the RTS-genre what the first Halo was for the FPS-genre. I don't believe it really, that would be too crazy- but it's possible.

Reconsider:
C&C meets Halo+ lots of Spartans mixed by Ensemble Studios= every thing is possible.

And there are so many AAA shooters out there but no AAA RTS. It' time...

Considering how poorly Resistance 2 sold and Motorstorm 2 . . .

I think it's a no brainer, how many Age of Empires games sold?  How about Halo?

Personally I liked age of empires, and hate halo, so I don't think it'll be good, but it will beat KZ2.

KZ2 is the test to see if anything can be a big hit anymore on PS3.

Actually, I think the real test for that is Gran Turismo 5.