400ex's are slow stock...but with a few mods they wake up nicely. My roomate has an 02, it was a dog at 1st, he put a full White Bro's exhaust on and I couldn't believe the difference. It was close to going from stock pipes to aftermarket on a banshee - night/day. I like his EX a lot, will probably get one or trade the g/f's shee for one if I find someone willing. They handle great & take some major abuse, and will run forever. + you can find damn nice used EX's for a lot less than you'll get a craptor, kfx(yuck), or predator though. Out of your choices, I'd go with the predator--as long as the seat stay's on you're good to go.
But like Barracuda said....if ya can, go with a yfz!