I had a UTEC on my car when I bought it. It's old tech man. There are very few advantages anymore to a piggyback vs reflashing the stock ECU. They're not well supported anymore and most tuners don't want to deal with them. All that said, they are very capable little boxes.
05 STi is got to be close to the most recent ECU they were released for anyway. Why the switch?
first reason why was the price. then 2nd through 8th of all was because of the anti lag, speed density, and the perception of the ease of use. with OS i havent seen a way where speed density was an option nor was the anti lag. also paying a really low price for the unit was worth it to me
UTEC can't do anti-lag (this requires a stand alone, not mention that it will destroy your exhaust system on a street car) and OS can do speed density. The UTEC is easier to use though, as it's more simplistic and has fewer options.