you have to look at the long run of your project. are you making a rally car what will really see some action, or is this just a dream that will never happen. if you are building a street car and want power then weight will not be as big of a deal to you.
a legacy turbo has about 500 more pounds then an impreza. that 500 lbs can really take its toll on you when you are in a rally event. if you are at the drag strip you will also have better gains, but its not as much of an issue because you are not cornering.
what do you want?
a legacy turbo is a rare find, but if you can get your hands on a good one, and put a little bit of work into it then you could have it blasting almost anything at the drag strip. bad part is, that suspension and aftermarket parts are way more expensive then they are for a impreza. you can get a sway bar for about 60 bucks cheaper for an impreza then for a legacy, and that adds up fast. and used parts, you could save a ton by getting used parts for the impreza.
hey serious, there is a guy on the legacy forum trying to get ahold of an ej22T, its in my post title is "I feel like a jdm whore"