Jet Trooper: In my opinion, he isn't very good, but to make best use of him, use his TDs/Grenades for groups (Mostly you'll want to ambush with this guy, so the thermal detonators are pretty good), his EMP for enemy emplacements and vehicles (And stronger enemies such as Droidekas), and use his Commando Pistol for basic defense. The reason your main weapon is a pistol is because he is made for swift ambushes.
Dark Trooper: In my opinion, he is THE way to go if you are looking to ambush someone. First of all, his jetpack doesn't fly for long, but it has a huge burst of speed for a sudden strike. Not only that, but a weapon that is capable of severe damage to massive amounts of enemies. If you charge it in the air, while ambushing a group, it will do amazing. He also carries Thermal Detonators to take care of tight situations.