Not to put the PS3 down but the game is using Unreal Engine 3, actually an enhanced version of it called Unreal Engine 3.5, and the engine simply runs better on the 360. Considering this game will likely push it harder than any game has in the past it could have nasty framerate issues on the PS3.
Also there's nothing stopping the game from being bigger this time around on the 360 as disc space != game time. The game could be on one dual layer DVD disc and still take twice as long to complete. And there's nothing stopping them from using 2 dual layer DVDs if they have issues with space for the game.
Finally it really wouldn't make sense for Epic to build the game around a blu-ray disc because the PS3 would never be able to sell as many copies of the game as the 360. The game simply seems to appeal to the 360 fanbase a lot more.







