01/02 had oil starvation issues in the bottom end. 04 the motor was redesigned, making the 04 and up bikes better. There’s more little tweaks and upgrades that made things on the 01/02 slightly obsolete. Front brake was improved in newer years, linkage change.. etc. I’m sure pit can chime in for more info.
With all the being said, I have owned a 02 and currently own a 06. I LOVED my 02, but I would go through multiple crank bearings a season. No issues with my 06. Whatever you do though, put a Hinson clutch in it, and save yourself problems with the stock basket. They’re soft.