Xanax (alprazolam) is more potent than Ativan (lorazepam).
Typically 0.5 mg of Xanax is equal to 1 mg of Ativan. However this does vary from person to person.
Xanax also works faster and lasts a shorter period than Ativan. So it is logical to say that Xanax is more "powerful" because of its potency and rapid action.