In the literal sense - a spark is a small flash of light - usually from an electrical discharge, or a flint, like a cigarette lighter. A trigger is a mechanism that initiates some action, such as the trigger on a gun. In a figurative sense (which is what I guess you are asking about, since you are using and comparing both words) - they can be used similarly, such as sparking a romance, or insulting someone to trigger a reaction. Spark would be used where there is an implication of flames or energy release, such as romance, anger, fires, chemical reactions