Which is better: to pray in a non-designated corridor on the second floor, or to pray in the prayer room designated for that purpose on the ground floor?

The places mentioned do not qualify as mosques. Therefore, it is preferable to pray with the larger congregation, as the Prophet, peace and blessings be upon him, said: "A man's prayer with another man is purer than his prayer alone, and a man's prayer with two men is purer than his prayer with one man, and what is more numerous is more beloved to Allah the Almighty." Jurists have explicitly stated that observing the greater congregation is better, unless the prayer is in a mosque with a small congregation, which is then prioritized over praying in a house with a large congregation.