Anything to do with racing I'd go with the R, that's what they are made for. The 400EX was a race quad in it's day, but that was in 1999 when there weren't many competitors but the Banshee and Warrior out yet. Once the big thumpers started coming out, the 400EX became a trail/off road quad from then on. A 450R will walk all over a 400 no sweat, with great handling and much better suspension to boot. Only downside to an R is they are much higher maintenance and don't have reverse, most are kick start too. They have a separate gear and crank case so different oils need to be used in each along with it being liquid cooled instead of air cooled.