Burnout Paradise and DMC4 had been in development for the PS3 long before MS secured a port for Xbox 360. It's no surprise that they should run better on the PS3.
But honestly, DMC4 looks a tad better on Xbox 360 than PS3 on the gametrailers video. I know it's not very reliable, but not bad for Xbox 360's version which started development later than the PS3 version.