STi without a doubt. Much stronger engine with forged pistons. Bigger turbo. Bigger intercooler. Brembo Brakes. Better stock suspension.
ECU and
TBE and you can get 200kw ATW reliably. A normal WRX with 200kw ATW wont last any where near as long as the STI. WRX ej205 is not as strong as the STI ej207.
some people dont like it but I like the STI Suede Seats better.