can't go wrong with either bike. i have owned a 2004 550 and 650 and ridden them back to back many times trying to decide which i liked better. i ended up selling the 650 for a few reasons. my 650 definitely felt noticeably heavier at the bars than my 550 and i ride more tight stuff. also my 650 was worth quite a bit more. if i rode mostly open desert type stuff i would of kept the 650, it felt a little more "planted" and stable at speed and that grunt is insane, not that the 550 doesn't have grunt, but not like the 650. both awesome machines. sold the 650 with 3350 trouble free miles. my 550 has over 8000 miles 250+hours and has never let me down. i just replaced the cam chain before moab, no failure, it was just time. just my 2 cents and experience. good luck
cheeseberger