Which is the correct comparison between «كُتُبٌ» and «مَسَاجِدُ»?

Diptote Causes — Ultimate Broken-Plural Patterns

• Kutub is fully declinable; masājid is a diptote
• Both are diptotes
• Both are sound feminine plurals
• Masājid is fully declinable; kutub is a diptote