Things to think about:
WRX: Yes Direct Injection is great (look at the BRZ) high compression high torque at low rpm but it's a 1st year engine.
6 Speed in the WRX is out of the TY75 Tranny, just with an extra gear, it is in no way as strong as the STI gearbox.
Timing Chains are not the best for motorsports, but really good (if you change your oil often and religiously) for street. Chains stretch unproportionally, meaning a link or series of links can stretch and not others, which makes it hard to control cam timing. (yup, this is reality). But for the street, it's low maintenance and no worries, unless you wear out the chain guides.

STI:
2.5L engine, yes it's the older engine, but it's proven with lots of good history. When running a car "Stock" there is no problem with this engine, unless you bang it off redline all the time.
Timing belts is safer for motorsports use, they are more uniform and have very low failure rate, very low friction as well.
2.5Litre Engine is mature, not a 1st year engine, with tons of aftermarket support and all the known issues have been identified and there is an abundance of aftermarket support.
Chassis is the same as the WRX, but spring rates are different so are other components, like spherical bearing front control arms for more direct input and control.
The 6 Speed Tranny can take whatever abuse you can give it, if SteveD has not broken it yet, it will not break.
Brakes, we all know the first thing to change is the brake pads for motorsports use, I personally use Stoptech brake pads and they are ok for light track duty. There is also less flex on the front hub on the WRX's, necessitated by the use of solid mounted calipers. The front end of the WRX still uses floating rotors.
There's more to it than meets the eye, at the end of the day, you'll need to give me a call so we can get you a deal on your car, plus you can take advantage of the CARS 750 cash back if that still exists, since you've been a member for a long time.