Well here I am. Pondered the 390 vs 450 question and asked a lot of people. Some of whom may have actually known what they were talking about.
Basically I'm 100kg, do trail riding with some technical stuff and some open fire trails. I believe that I would have enjoyed the 390, but occasionally would have liked a bit more power for e.g. heading up a long hill and using just a little more grunt to get over erosion banks, and lofting the front wheel across ruts, logs etc.
I've had bigger bikes in the past, and sometimes had to spend energy being cautious about all that power.
So I went for the 450. Time will tell. (you can enjoy a ride on a TTR250 and on a TE610, so what does it matter really!)