I'm the other way round to Warthog in that I've had a 600 for the last 15 yrs but never ridden a 400. I've used the 600 for just about everything - green laning in the UK, touring all over EU, two trans sahara trips etc.

For muddy tracks etc unless you're some sort of moto god the 600 is too big and heavy. It may only be 8kg more than the 400 but after a while you really feel those kilos. I'd take an XR250 over the 400 and get the weight down even more.

For normal winding roads the 600 is a great bike. No great top end but the punch out of the corners makes for an enjoyable ride. I used it to travel to work in London for years and it's great in the traffic - until you stall it at lights when you look a complete plonker having to get off and push it to one side.

Loaded up for solo overlanding it's ok (ish) I didn't do much to the subframe other than redo the welds at the end where the mudguard mounts but I did weld in some extra brackets at the bottom so I could triangulate the pannier frames. It hasn't broken so far but it does flex a lot side to side and I've never really been comfortable with it.

The biggest problem with the 600 is the kickstart. I've broken a couple but it's really the whole palaver of getting the thing started. You can't just kick and hope like you can with a two stroke. It requires a ritual that you have to go through and a kind of whole body commitment that just becomes too much after a time. The 400 has to be better in this respect. On the last UK - Gambia trip I got to the point where I'd leave it idle rather than have to go through the starting ritual and when I really had to turn it off (borders / hotels etc) I'd pay the (inevitable) bunch of kids 20p to push start me.