The most accurate response comparing the reactions of the Byzantine and Sasanian Empires to Islamic conquest is:
The Sasanians crumbled due to instability, the Byzantines held out despite losing territory.
This statement reflects the reality that the Sasanian Empire was weakened by internal strife and had already been significantly strained by its conflicts with the Byzantines, which made it less able to resist the rapid expansion of Islam. In contrast, the Byzantine Empire, while losing significant territory in the face of the Islamic conquests, was able to maintain control over parts of its territory for several more centuries and eventually recover to some extent, continuing to adapt and survive as an entity.