I don't know if it's possible to find specific details in the epic poem of Beowulf that indicate that one ruler is better than the other.

During the time Beowulf visits the Geats in order to defeat Grendel and his mother, Hrothgar is an honorable man, but not a warrior. He is a respected leader who cares deeply for his men. He honors Beowulf's willingness to travel so far to help the Geats defeat the monster that has plagued them for twelve winters, and honors and reward Beowulf for his help.

It is not until approximately fifty years later, when all of Beowulf's kin have been killed, mostly in war, that the title of king falls to Beowulf. He is much older, but the heart of a warrior still beats within his breast. When the thief disturbs the lair of the "fire-breather," Beowulf is quick to gather his men to defeat the creature in order to protect his people. He is honorable and faithful to his subjects, but perhaps things are now different over the passage of the last fifty years, for when confronted by the "fire drake" (or dragon), Beowulf's men flee in fear—except for Wiglaf, who displays the same code of honor and valor that has shaped and governed Beowulf's life.

Both men are loyal to their people, willing to do what it takes to protect them. I cannot see that either man is the better warrior, but that the times have changed and the men Beowulf depends upon are not made of the same "stuff" as were Hrothgar's men, many years before.