I have decided that I am going to go the aftermarket standalone ecu
It is easier to use a aftermarket ECU because of the wiring and tuning requirements of the stock (US)ECU. But not easy. Remember you still need a harness that mates to the bulkhead Subaru harness for running the rest of the car. If you make that change yourself it is not expensive, but costs a lot for outside labor.
Still, it can be and often is done by the enthusastic person wanting a Subaru. I have done it both ways, first with the Suby computer and an Apexi thingy, now an Autronic M2. The Autronic is much superior but cost $1500 plus the wiring expense and tuning. Expect to spend $3000 if you pay to have it done for most any standalone. The cost of boy toys I guess.
If you are going to make the sacrifice to get your car going, on balance a standalone is the best route. I have heard that TEC2 is not a top level computer, but it does cost less and many people use it. Like the man sez, you get what you pay for. Success.