Personally, don't know much about the differences between the two. But youtube has enough videos on it.
Edit: forgot big difference, according to the vid 250s are carbed, 300s are EFI.
Yeah, EFI vs carbed...but I never had issues with the carbs. Even with running the gas tank low on fuel all the time to save weight/handling feel on the track. There is no fuel filter either. You can put one on but make sure it flows fast enough or you run out of gas with a full tank of gas.
Component wise...
- 300 can be found in ABS and non-ABS
- 140 rear wheel - but good radial tires still come in 130, so for street it doesn't really matter.
- Nicer digital dash. 250 still has a fuel gauge.
- mechanical speedo on the 250...doesn't really mean much
similarities:
- front and rear suspension is the same
- swing arm
- handle bars and controls are the same
- frame is mostly the frame except for a few different mounting points and brackets
- brake callipers are the same...even on ABS model. The hoses and ABS pump/electronics are the addition
- rotors are same size just look different/more flashy on the 300
- foot controls are the same. Body position is pretty much the same...only difference is the seat and how the tank feels between your legs.
- a fair amount of engine parts
- coolant system
- front wheel is same size. Swappable if you have the speedo drive
- chain and both front/rear sprockets
- triple tree/yoke
- ignition/locks
....I'm probably forgetting lots of small stuff but that stuff doesn't really matter anyway.
EDIT: actually, the rear shock might be a different shock but same length and quality level.
Front's are definitely the same thing. Slight different casting on the lowers. Same internals and sanctions.