I've ridden a KX 250 with all the woods mods (flywheel weight, ims tank, steering damper, softened suspension, etc). Fun, but I would take my KDX with the rb mods and KX forks any day over the KX. The power delivery is just different. You can go as fast if not faster on a KDX, and it's easier to ride. This is my analysis for tight woods and/or single track.
As a former mx'er I will say that If the track opens up and your dealing with burns, whoops, doubles, etc, give me the KX.
I prefer to make a slower bike faster vs making a fast bike slower. And then there's the comfort in the saddle. Sometimes I forget, then I switch rides with a friend and it quickly becomes evident that I appreciate the seat foam on my KDX.
For what it's worth.
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