True Polymorph is a lot stronger and has more variety than its regular version. Not a big surprise if you know that True Polymorph is a level 9 spell, while Polymorph is only level 4.  Unlike the regular version, if you concentrate on it for an hour, the spell lasts until it is dispelled. The target's transformation is also not restricted by just beasts. You can change a creature into an object, or an object into a creature.