I don't own Modnation Racers but I do have both Blur and Spilt Second for the 360. For me, Split Second is by far more fun. It has a much better sense of speed and is a far more enjoyable experience than Blur. Split Second is non-stop action and is good for about 20-30 hours of gameplay in the single player mode. Blur is a kart racer with more realistic cars. It's not a bad game but I was expecting a little bit more from Bizzare Creations. Both games have decent graphics. Finding an online match in either one has proven quite difficult. Both games have a decent variety of cars and race types.
So I guess if you're looking for a good arcade racer, get Spilt Second. If you want a kart racer with realistic cars, get Blur. But my opinion is Split Second without hesitation. It's the better of the two.
