I have had both. The 600 delivers most of its power right from idle to mid range which makes it a bit of a hand full on steep hills, all it wants to do is stand up on the back wheel with just the slightest turn of the throttle. Its a bit like a tractor..
The 400 delivers most of its power mid range to top end which makes it a dream on hills, even if you get into trouble half way up a hill and have to button off, you just hit the throttle and it will just keep climbing. They just love climbing hills.
The 400 is a lot lighter and slimmer than the 600. In my opinion the 400 is the better bike by far.
They apparently had a slight problem with second gear which was rectified after the 2000 model... I have a 2001 which I have had from 2002 without one issue.