Upgrading to a 120GB hard drive on an offline Xbox 360 depends entirely on how you play. Here's the breakdown to help you decide.
Buy the 120GB drive if: You primarily play single-player campaigns, want to install disc games for speed/quiet, enjoy OG Xbox backward compat, and accept the offline ceiling.
Skip it if: You crave online play, want the best versions of multiplayer-heavy titles, or would rather put that money toward a Series S/X (full backward compat + Game Pass + online).
For a dedicated offline 360 station, the 120GB drive is a quality-of-life upgrade. For anything else, look forward.