I own a 520 EXC, WR 400 and a 300 EXC. My buddies mostly ride 400 EXC's. All are great and have their advantages. But living out here in the wide open spaces, if I could choose only one (drum roll please) the 520.
Feels light and nimble like the 300, has the juice for the sandwashes and just doesn't tire you out like the 426's hit will. (Not to mention the starting)
If I lived in a place with lots of trees I would go for the KTM 400. For the desert the 520 is the one, even for this tired old war horse. Makes a fella feel young again.
The older I get, the faster I was.