The difference between Christianity and Islam starts at the top:
Muslims are told that Muhammad - a slave-owner, sexual glutton, thief and killer - is the most "beautiful pattern of conduct" and "example" for mankind to follow (Qur'an 33:21), as well as the "exalted standard of character" (Qur'an 68:4).
Christians are told to emulate Jesus - a pacifist and servant - and "walk, even as he walked" (1 John 2:6). Unlike Muhammad, who ordered military assaults against Christians, for example, Jesus told his followers not to resort to violence and to pray for one's enemies.
These two men could hardly have been more different in how they lived or in what they taught others. Why should we not then expect starkly contrasting legacies - from the conduct of their closest companions to the livability of modern-day countries influenced by the predominance of one founder's teachings over the other?
As Wafa Sultan (who describes herself as a Muslim who does not adhere to Islam) puts it: "The problem with Christians is they aren't as good as Jesus. But thank God most Muslims are better than Muhammad."