What do you want to do with it? Trail ride?
Is the Katoom an EXC? If so, it makes an excellent trail bike, but it is a big heavy bike with tons of power. Not the best for single track, but it can do it.
The YZ is a motocross bike, will be a handful on the trails.
I had a '02 520 EXC, great bike. Very reliable.
I don't know karate but I do know ca-razy.
'06 Husqvarna WR125