彼の部屋は汚い___、物置のようだ。
Q. Which phrase completes 'not so much dirty as (a storeroom)'?
Translation
Calling his room dirty is an understatement—it's more like a storage room.
Explanation
AというよりB corrects a description: 'B rather than A'. のわりに and にしては compare against expectations ('considering'), and ばかりでなく adds ('not only').