The thing with weapon overheat on Insanity is that the duration is very short. If you pay close attention what you will see is the enemies will fairly quickly reload / pop a thermal clip so that they can fire once again. Synthetic stun duration is a bit better, and more noticeable, and is also why I like a couple ranks of Disruptor early game on a soldier. Makes the mechs you fight early a little easier to deal with.
What is better is up for debate. In a vacuum, squad Incendiary might be slightly better, especially if you are running Miranda / Garrus w/ Dual Area Overload. However, on a soldier what you are really deciding between is Squad Incendiary vs Inferno + Sq DA, and in that case I would say the latter is typically better.