Back when I had the V6 I installed a STB, the GT came with one stock.
Before the STB the car felt like the front wheel on the inside of a turn was going to lift off, the whole car kind of floated. After the STB the whole front end felt a lot more planted and handling seemed much better.
As far as which STB is better, you are on your own, I only ever had the OEM Boss/V6 STB.
2011 V6 Mustang - Premium 202A Spec (Pony Package) - SOLD!
2014 V8 Mustang - Premium 400A Spec
6 Speed Manual, 3.31 Rear
Black, Charcoal Black
Comfort Package, Security System