The most accurate response comparing the growth of Islam and Christianity in Europe between 300 and 1500 CE is:
Both religions were widespread on the continent at different points in time. Christian empires flourished in Spain under Islamic rule and during the Ottoman Empire. Islamic kingdoms flourished across most of Europe apart from these areas and regained a foothold in Spain after the Reconquista.
This response captures the complexity of the religious landscape during this period, highlighting the coexistence and influence of both religions in various regions of Europe.