I ride almost solely technical trails - tight technical trails, first and second gear technical trails.
Now I haven't ridden the 450 or the 390, but I have talked to some people who have ridden all three bikes and they say there are handling differences, *but* many preferred the 570 and don't feel it is a beast or at a big disadvantage on trails. The general consensus is that over a long day on the trails at a fast pace, the 390 is better because it handles a bit better and is less tiring.
Personally, I run my 570 on the 'soft' map with a 12 tooth sprocket and a low idle in combination with the Rekluse. For a street legal dirt bike I think it works very well. I suggest riding all three bikes on trails and then buying the one that you like the best.