No no, get the STi. It's a much better platform and finding one clean that age is extremely hard (I could not). The transmission alone is worth the price difference but you'll also get a 25% larger motor, brembo brakes, forged wheels, better suspension, bigger turbo, bigger intercooler, etc etc etc. It's the best rounded $6,400 upgrade you could ever do to your WRX.
You might try to sell the WRX privately though instead of trading it in. If it's clean, that would be a very easy sale. A lot of people want clean bugeyes, there aren't many of those either.
2015 STi: Still Stock
04 STi, 04 WRX: SOLD