sti is more powerful,lighter, better suspension ,better brakes,has strut braces ,different interior,6 speed box,uprated diffs and shafts,revs higher,and more just cant think of what else im sure someone will tell ya
No doubt about it, the STI is better!
The most appearant difference is the extra 60bhp (220bhp to 280bhp)
You'll not really notice the brakes to much and a simple spring swap will sort the suspension!
You'll get a WRX that is 2 or 3 years newer than an STI
So better older STI, or newer fresher WRX
Dan