What kind of racing do you plan on doing? I have both machines. I've raced both machines as well. If you are planning on XC racing, I'd keep the XX and get some suspension and stuff for that, to make it handle better for racing. I'd keep the XX anyway for trail. If moto, then the 450 will dominate. However, it will not be a trail machine if you set it up for moto. This is all saying you are going to MOD it up for the appropriate racing. I've done both moto and XC. You'll have to lower it for MX, widen it up, and get super plush suspension. For XC, most guys lower it a little bit while widening the front just a tad. In either case, neither machine pictured will be all that well with stock suspension. So you are looking at that factor. The 450 has more companies that work with it, so more parts. But I'd def say keep the XX for trail, and if you're looking at XC, you can compete the XX at almost all levels. If you got really good, as in riding with top riders, there is restrictions on motor size (right now for instance with GNCC it is 450cc for expert class quads). That would be the only downfall. Trust me, if you are older (than say 25) you probably will not compete in any expert class ever, as the guys in expert now start when they are barely old enough to walk. So the engine size wouldn't be a worry. The other benefit with the XX is the lack of maintenance on it. The 450 you are suppose to check valves after every 15 hours, plus you'll have to change the .7L of oil on each engine and gear side after every ride (should anyway).
2008 700xx; Power Bomb header with power core 4 exhaust, Power Programmer III, pro armor nerfs, prm products control front and rear bumpers and full skid, 10" ITP rears, ITP Baja's fronts, RZR 2's all the way around, Uni filter, 13t sprocket, CCP steering Damper, DS450 shock swap, Dual Gasser Throttle, bar risers and Moose barkBusters. 730XX kit plus other engine mods!