I was wondering what ECUs you all are using on 2.0L STi and Spec-C engines? I am hoping to get a V7 or V8 RA Spec-C engine to swap into my 02 WRX sometime this year. I want to retain the AVCS system, but am looking for an aftermarket ECU so that I can have better tuning options. Right now, I am thinking about using the Stock ECU for AVCS and using a TEC-III for the rest of the engine management and ignition. That setup is kinda pricey though, and I am not quite sure if the TEC is really as good as the advertising makes it seem.

use the stock ecu and add the avcs "kit"

There are NO ecu's better then factory. Dont get a Tec 3, just flash the factory ecu. Doing that will save you many sleepless nights :D

Or, use a Hydra.

Factory.