Rode all 3 on the weekend in tight and steep forest type riding. In the tight stuff the 400 and 520 felt similar though the 400 gave a feeling that it is easier to ride for a longer time. At low speeds the 520 wants to jump of the clutch in a stronger manner coming around corners or out of pot holes and over tree roots. The result is it is a bit more tiring than the 400. The 400 seems to have a slightly more "plug along" type of feel over the 520. If I had to give a percentage of how much better it is in the tight stuff I would say about 10-20% depending on rider. On the more open trails going back to camp the 520 is easier to pop wheelies in 2nd gear to practice stand up wheelies :-) where on the 400 I felt more comfortable doing a wheelie in 1st and trying to snick it into 2nd while standing up :-). The 520 is the hoon machine. The suprise came when I tried a mates KTM250. IN the tight stuff the 250 really turned where you pointed it. It felt very balanced where the two 4 strokes felt heavy and much less precis! I dont think there is much difference in the weight but the 2 stroke I think carrys its weight lower over the 4 strokes? The 250 felt like it was turning around the centre of the bike very neutral where the 4 strokes felt like they were "pushing" into corners, wanting to run a bit wide. If the 2 stroke 250 had an electric start I'd be sold ! But at the moment I think the 400 is the best of all worlds.