I get a feeling from various observations, Islam is a very weak religion and it will crumble in the long run. Christianity is much more adaptable with it's tradition of criticism and interpretation. Plus it is more appealing with the story of the life, death and resurrection of Jesus.
Right now Christianity is winning new converts everyday, (for example Korea has become 1/2 Christian in just a few generations, China's Christian community is rapidly growing), Islam's numbers are growing generally via population growth in existing Muslim communities.
I read something like 6 million Muslims convert to Christianity every year, they keep it secret to avoid reprisals from people they know and the authorities.
In the long run Christianity in some form will become the predominant religion of planet earth, Islam will decline and be restricted groups like the Sufi's. Buddhism, Hinduism, Confucianism will be just marginal religions compared to Christianity and any new age religions which come into the fore.