i rode a sherco 450 last year at a sherco ride day, seemed like a nice bike but i haven't done a lot of research on them. it feels small and nimble in the dirt but personally i felt it started to feel a bit twitchy at speed, unusual for a big thumper.
i'd agree with the comment above and say go for the bigger engine if you'll spend some time loping along sealed roads. a few years ago i had an FE570 that doubled as my adventure bike and woods bike.
the engineering behind the beta is very solid, the only issue that's cropped up really is just those plastic oil pump gears. i think the recommended replacement interval is 100 hours although some guys have gone way beyond that. when they are due just put in aftermarket alloy ones and forget about them.