It has been some time since I reviewed the primary sources so please correct me if I'm wrong, but I believe that the Chronicles of both Jean Froissart and Geoffrey le Baker, as well as a couple others, describe the prince as having every intention of fighting the battle at Poitiers. Furthermore, considering the outcome of the battle and the extensive preparations taken on behalf of the Black Prince's army, the notion that the Black Prince had always planned on confronting King John II isn't too far-fetched. For those of you unaware of the outcome, despite outnumbering the English ~3:1, King John II was captured and the French army was thoroughly crushed.
However, in a letter written to his father, Edward himself states that he was attempting to retreat from King John's forces. Likewise, the movements of his army in the weeks preceding the battle imply that the Prince was seeking to return to the safety of Bordeaux. The Black Prince's victory at Poitiers can be seen as evidence of his brilliance as a military tactician. Considering the sheer size of King John's army, and that Edward had no way of knowing how poorly organized and undisciplined it was, it seems reasonable to conclude that, as a military commander, the best option would be a tactical retreat.
I'm interested in hearing why historians, in their own opinion, either believe the Prince was or wasn't retreating.
Are you referring to the letter Prince Edward sent to the City of London following his victory? Edward doesn't exactly say he was planning on retreating. Instead, he says he was withdrawing to link up with the Duke of Lancaster after abandoning an assault on Tours. After he rejected the French negotiators, Edward waited at Chatelleraute for four days in order to determine where exactly the French king and his army were. After that, it was in fact Edward who was pursuing the French, rather than the other way around. After that, the campaign was a messy series of maneuvers and skirmishes as both armies attempted to locate the other. Edward was clearly intending on having a battle, but it would be on his terms and on terrain carefully chosen to protect his men from the French attack.
The Poitiers campaign is (in terms of strategy) not very different from the Crecy campaign, or the Black Prince's 1355 chevauchée (although in that case, the French refused to come out to fight in the field). It is a common misconception that the English engaged in chevauchées in order to avoid confrontations with larger French armies. Most sources point to the exact opposite scenario: the English actively desired open combat in the field, while the French only attacked marauding English armies when they felt they had no other choice. The English knew that their armies were highly effective in the field, and frequently won battles against numerically superior foes. The prospect of engaging larger forces was not a cause for much fear, provided that the English were able to maneuver so that they 1)fought the battle on favorable terrain and 2)prevented multiple French forces from enveloping them. What has often been interpreted in the past as English armies being chased by French armies is more often English armies attempting to maneuver in order to pick the best location for open combat. For more on English military strategy in the opening stages of the Hundred Years War, I highly recommend Clifford Rogers' The Wars of Edward III: Sources and Interpretations.