Just to make it clear: this is not a Motorstorm ripoff. Yes, it features off-road racing. But, you have only ATVs to ride. Although, unlike MS you can customize your ATV in order to suit your driving style. You can not do it in MS, you can only chose vehicle type there.
Furthermore, you can perform tricks in this one - and this is integral part of the as tricks will give you boost, which comes for free in MS.
As for graphics, its extremely good. Myself being a software engineer and knowing PS3 specs, I am sure that devteam tried hard to pull off this level of detail. But I have to admit, that lack of split-screen is frustrating.
In short, if you are chosing between this game and MotorStorm, definetely go for MS. But if you are a racing fan then this one comes recommended!