you can get a new 09 wrx in that HP range very easily. a turboback exhaust + a custom tune will get you there without really sacrificing drivability or reliability (depending on your driving habits)
the 06 sti will have a rougher feel to it. you'll feel every bump in the road. it's a raw in your face kind of power.
the 09 wrx is a little more refined. the interior is beautiful. the drive is quiet. the power is there but it's not as in your face as the STi will be.
i would drive both of them and see what you prefer.
there are also a few other factors you'll need to look keep in mind:
is a warranty important to you?
finding a stock, unmolested 06/07 sti will be VERY hard
body styles are completely different. which one do you prefer?