Your car will be a big tradoff on what you choose. To add on to what wip99gt said...which is absolutely true:
The FI engine of the WRX is more resistant to altitude changes in terms of loss of torque and therefore translating to horsepower loss. HOWEVER,
There in this world...what it ALL comes down to is 2 types of engines. Just 2 types. One type makes horsepower by displacement, the other makes horsepower by rpm. Engines that create horsepower by using less displacement and higher rpm use gearing to become just as effective as those with more displacement and less rpm. However there is one very big downside, and that they are extremely vulnerable to weight. Sudden increases in weight...like passengers will slow it down a lot. If a bigger displacement with more torque but equal horsepower suddenly has a weight gain...its torque will act as a buffer and you won't notice much of a difference. However, in high revving engine with minimal torque...any sort of weight will drastically slow it down...even if it is still fast, it makes a noticeable slow down.
Turn on your AC in a 2.0L 200hp car, and then turn on your AC in a 5.0L 200hp car. In the 2.0 car, you will probably see a noticeable loss of power, while in the 5.0L car you won't notice a damn thing. Thats torque acting as a buffer for your power.
The other side of the argument is the Legacy 3.6R has a NA engine and thats going to get chocked at high altitude while the turbo on the WRX will keep up better. Its a hard decision and I don't know what to go for in this case.