Thus, the two author's viewpoints are different because of the purpose and scope of their works, and not necessarily because one is a "better" or more thorough researcher than the other.
The evidence each author uses does support their arguments very well. Each man chooses the evidence that is most compelling and includes it in their assessment of the man, and this helps their works seem more credible and believable. Both use anecdotal evidence, (such as the story of McClellan snubbing Lincoln), but they use quantifiable evidence, as well. For example, McPherson makes assessments of McClellan and his military decisions, but backs them up with maps, battle positions, and hard evidence that shows how each battle was fought, and ultimately won or lost.
Both authors paint compelling portraits of McClellan as a general and a man. It is clear he was a controversial and complex personality who made some disastrous decisions while he was in command of the Army of the Potomac. Both authors also have serious research and backup for their conclusions and interpretations. If any interpretation is more convincing, it would be Rowland's, simply because his entire book is devoted to discussing McClellan and his generalship, while McPherson's is not. McPherson discusses McClellan in relationship to the entire war, while Rowland discusses McClellan in relationship to himself and his command. They are different perspectives, and so, they achieve different results. Neither is a poor interpretation, they simply serve different purposes, and so, they achieve different aims.
It seems that Rowland's account is more interesting because it is more detailed, and because it takes more time to attempt to explain some of the historic suppositions about McClellan, his personality, and his leadership. On the other hand, McPherson's is a great overview of McClellan's rise to power and subsequent fall from grace. Each serves a purpose, and reading them both together gives a more complete picture of McClellan and his decisions during the war. It is quite clear that Rowland wants the reader to make up their own mind about McClellan and his effectiveness, and that he does not buy in to all the generalizations that have been made about his mental health and fitness for leadership. He wants to get to the bottom of McClellan's actions, while McPherson is reporting them, instead of deeply analyzing them.
Rowland takes a lot more time to discuss the decisive battles and failures the general made, and he uses the viewpoint that McClellan's strategies may have been flawed, but there may have been good reasons for some of his failures. He is not so critical as McPherson is, and the detail in his work, using largely quantifiable evidence culled from McClellan's own papers and a variety of other sources. For example, he writes of the Battle of the Peninsula, "To Ellen, he wrote that he had just begun to train his artillery at Magruder's fortifications when he 'received the order detaching McDowell's Corps from my command-it is the most infamous thing that history has recorded.... [T]he idea of depriving a General of 35,000 troops when actually under fire!'" (Rowland, 1998, p. 108). Of course, Rowland uses anecdotal evidence, as well; otherwise both histories would be quite dry and uninteresting. Anecdotes keep the text moving along and the reader involved in the action, and both authors employ them to good effect in their works. Their evidence is compelling, their works are believable, and reading them both together gives a much clearer picture of General McClellan and his career.
In conclusion, both of these texts provide fascinating glimpses into the personality and foibles of General George B. McClellan. They show he was a troubled man who was often afraid to act, even though he fancied himself a capable leader. Perhaps he was not the worst Civil War general on record, but his command was taken from him, and his accomplishments were actually quite few. He was a troubled man, and had he been relieved of command earlier, the war might not have taken quite so long to conclude. It is impossible to guess what might have happened had McClellan been relieved after the Second Battle of Bull Run, but author Rowland notes, "Under the stressful and anxiety-provoking conditions that a command position entails, certain people react better than others" (Rowland, 1998, p. 234). Clearly, McClellan did not react as well as he could, and that could actually have led to the Civil War going on much longer than necessary, which is a tragedy for the United States.
McPherson, J.M. (2001). Ordeal by fire: The Civil War and reconstruction. New York: McGraw-Hill.