While there is certainly many ways to answer this question, Stephen Tomkins in his recent biography of John Wesley reflects eloquently on the fundamental difference between George Whitefield (a strong Calvinist) and Wesley.
The fundamental difference between Wesley and Whitefield was not the theological one that so exercised them both and sadly caused conflict in proportion to its inscrutability rather than its significance. They were both saving souls and whether that meant harvesting the elect or convincing the free made no difference to how or why they did it. The great difference in reality was one of job description [I would add the related point of a difference of personality]: Wesley was a preacher, pastor, leader, administrator and an architect of religious organization; Whitefield was a preacher. Although he founded some successful ‘tabernacles’, he had very little interest in organizing converts and left this mostly to others.
What is your response to this assessment?
I for one find the whole debate between Calvinism and Arminianism overheated. And I think one’s personality plays more a part than is admitted in what makes one predisposed to one side or the other of the proverbial aisle. What’s more, I think Tomkins is correct that the effectiveness of ministry is unrelated to theological position on the question. For ministry effectiveness the question of free will or predestination is a moot point!