The most accurate statement 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. Islamic empires flourished in Spain under Islamic rule and during the Ottoman Empire. Christian kingdoms flourished across most of Europe apart from these areas and regained a foothold in Spain after the Reconquista.
This reflects the historical context where both religions had significant influence in different regions during different periods. Islam had a major presence in the Iberian Peninsula through Al-Andalus and later, during the expansion of the Ottoman Empire, while Christianity became the dominant faith in most of Europe, especially following the fall of the Western Roman Empire and the subsequent establishment of various Christian kingdoms. The Reconquista, which concluded in 1492, marked the return of Christian rule over Spain, illustrating the dynamic and sometimes contentious interactions between the two faiths during this period.