[Bushwacker]

If you just want to lift the stern, tab location doesn't matter. However, trim tabs are often used to level the boat side-to-side or sometimes roll the boat away from the waves when they're coming from off the port or stbd bow to increase deadrise relative to waves and improve ride. If you put them on outer panel you'll have more leverage and won't have to deflect the tab as far to create a given rolling moment. Trim tabs create drag when they generate lift, so generally you want to use as little tab deflection as possible for most efficient operation. Bottom line: put 'em on the outer panel!