It's a blessing the game allows you to be both classes at once, rather than forcing two separate playthroughs for each, like tends to happen in many 'choose a side' games. Rivals divides the world into two factions, Cops and Racers, and allows players to freely jump in between them as they rank up in each class and unlock new cars, upgrades and weapons. I've spent the last few days with both the PS3 and PS4 version of Rivals, which are more or less the same game with the obvious power increase of the PS4 (which honestly isn't all that obvious in gameplay, and the screenshots in this review are dramatically better than the reality on PS4).
