Was Christianity a positive factor in the lives of the slaves? What were the differences between the Christianity preached by the slave owners and the religion fashioned by the slaves themselves?

This is a complex issue; as the second part of the question suggests, the Christianity that was understood and practiced by enslaved people was different than the version prescribed for them by their masters. However, religion was a major source of agency for enslaved people. Generally, their version of Christianity emphasized liberation and freedom. It also bore the influence of African religions, with various customs being preserved in Christian contexts. Slave Christianity provided what one historian has called "psychological refuge" for its adherents, who were faced with degradation in almost other aspect of their lives. As for Christianity as practiced by whites, its effects on enslaved people were mixed. In the South, Christianity was used as a justification for slavery. In the North, on the other hand, Christianity was a major impetus for the abolition movement, which had its roots among Quakers and other groups who emphasized that slavery violated the dignity of men. Many of the evangelical denominations actually split over the issue of slavery during the antebellum period. 
http://nationalhumanitiescenter.org/tserve/nineteen/nkeyinfo/aareligion.htm