you are right, You will have more freedom with the street tuner, because it's usertunable, and ECUtek right now is not (although they keep talking about a user tunable version, anytime now. . . ). Even if you never wanted to tune the thing yourself, or never wanted to datalog (which is sooo easy with street tuner BTW), or never wanted to monitor a live ECU in action, You can still bring the ST software to any reputable tuner and they will be able to use it to tune your subaru in ways they've never thought possible if they've never used this type of program before. I say get the street tuner, learn the basics about tuning and about your car with it, and then take it to a pro tuner.
I personally wouldn't spend 800$ on maps that were generated for someone elses car. They might work fine, they might work like dog crap. Are you willing to spend 800 smackers on that kind of gamble?
Lastly, A vf34 setup will generate WHP numbers in the mid to high 200's, then divide that estimate by .76 (76%, assumes a 24% drivetrain loss), and you get your ~375 BHP.