Forza does look good in motion, but when you see screen captures of it they almost pass off as real. When you're playing the game it is much more obviously not real. It's like the textures are mindblowing but the engine is only above average.
Motorstorm on the otherhand is kind of like the opposite. If you see stillshots the textures of the cars aren't anything special(though the scenery is amazing). Playing it firsthand, the way the game engine works, the physics, makes it seem very real, when ironically the gameplay is arcade-like as opposed to Forza that's a simulator.
Hope that explains it. Forza is an great looking game, just the screenshots make it look a little better than it really is.
Just to show what I mean...
I looked at this and I thought to myself, "Holy shit! If someone told me that was a picture of a real car, I'd have believed that."
Motorstorm on the other hand...
Looks great but not real.
This isn't at all a rip on Forza, rather a compliment to their textures that it's one of the few games that actually looks better as a screenshot than in play.