Installing the 2-pot rears requires that you get the whole hub assembly, as the parking brake drum is inside of it. My hubs, and calipers, are on their way over right now.
The vented Legacy rotors are also a very good option though, and I would say they are likely to be the best bang for the buck. I just like taking my car apart all the time, you know.
But in all seriousness, if you are taking your car to the track and/or you have heavy modifications (i.e. turbo), I would consider the 2-pots. I doubt that Subaru specified the 2-pot rears on the STi cars just for kicks, so if your car is performing at close to STi levels or beyond, it is something to think about.