I'd go with WRX. If you avoid the earlier years you end up getting a more stout transmission (than previous wrxs), more syncros, and if you get a late enough model, more displacement off the bat.
The impreza has more aftermarket parts, more companies that specialize in tuning them, and the same suspension configuration. You get AWD (which can be a bonus depending on where you live, it's not in your profile) and a rear LSD.
The bottom line is that both cars ARE brittle. How many 2.0s have blown on this forum? How many lsds have gone out the window? And on top of that, how many WRX owners do you know who have shot their transmissions? You cannot put reliability into the equation here as your key point.
So lets try to list the points and counterpoints.
MSP:
Lighter Chassis
Stock suspension parts are better
Better brakes from the factory
Better stereo (meh, some people enjoy their tunes)
WRX:
Larger Turbo from the factory
More available and mature aftermarket
AWD (could be a bonus, depends on how you look at it)
More HP gains for the $$