personally i do not own a 600cc bike.. my 300 is my first bike
i have ridden my friends old school ninja 636 (2004 i believe)
there is no better thrill than the scream of a 4 cylinder between your legs, the high revving engine and the high end hp. i can't wait to move up to a 600
if you'll be canyon riding for a majority of the time on your bike, obviously it's your decision as to whether or not you'd rather a lighter more agile bike compared to a mid-weight bike to flick through the corners of the canyons
street/city riding, there is no doubt about it that the 300 is the winner
80% of the time i'm riding it is cutting between traffic and backstreets.
but the 20% of the time im on long rides with winding corners and mates, the 300 is loads of fun. you can adjust your line mid corner with a flick your legs and it's stable all the way through
i reckon you ask your friend if you're able to test out his 300, even if it's just a couple of corners. that's the only way you'll know i think