Both of these legendary kings feature prominently in the medieval romances and tales of adventure as related in the Matters of Britain and France; and in both sets of stories, the realm of Fairy and the influences of enchantresses are keenly felt.
In the stories of Charlemagne, there is a clear frontier beyond which dwells barbarians and pagans; this is the Saxon frontier, and the peers of the realm campaign across the Rhine with Charlemagne for a period of almost 20 years. For D&D purposes, it sets up a fantastic borderlands and wilderness, clearly demarcating the realms of Law from the realms of Chaos - there's a visceral sense of crossing a no-man's land when the Franks leave civilization behind and enter the pagan realm. In Three Hearts and Three Lions, the frontier is further mystified by placing the Twilight Realm there, the land of Fairy from which the forces of Chaos contrive to return the world to eternal gloom, banishing the sun.
Arthur's land of England has no such frontier of evil; there are conflicts with Saxons, the Orkneys, and Cornwalls, on the fringes, but encounters with magical realms happen nigh anywhere on a quest, usually when a knight is deep in a forest, or conveyed by boat to a strange place across water. Chaos is very much where you find it, as if commonplace and ordinary woods and lakes can transform overnight, thrusting a questing knight into contact with the numinous or the enchanted.
So here's a fine question for a Friday - in which legendary realm would you rather place your D&D style adventures? The enchanted forests of 5th century Britain, last haunts of the druids, or the dark, brooding woods past the Rhineland, home to pagan barbarians beyond the very frontier of civilization?