What does your mate want to do with the car (mod wise)?
The wrx/sti is a nicer car for a daily but the evo is a lot more fun. In terms of mods; the 4g63 is a stronger engine than the ej20/25 (the 25 even less so).
As for the gearbox comments, the 5 speed is the one with the short fuse but that's only when you start running loads of power (200kw atw, remember stock they're about 130kw atw) and abuse the thing. Show it mechanical sympathy and it's okay. The 6 speed from the STI is the strong subie box. For the Evo's the 5 spd is the stronger box.
I own a wrx and have driven an 05 sti and evo 9 back to back. I preferred the Evo; the only reason I bought the wrx instead of the evo was because of cost (as I wanted it new).