OnyX is the elder statesman of Mac utilities: free, respected, and maintained by Titanium Software for more than twenty years, with a dedicated build for each macOS release. Crumb is a modern, guided disk cleaner. They overlap on caches and cleanup, but they are built for two different kinds of user, and pretending otherwise would sell both short.
The short answer
Use OnyX if you are a technical user who wants free, direct access to macOS maintenance tasks and cache clearing, and you are willing to research what each checkbox does before ticking it. Use Crumb if you want the machine cleaned with explanations, approvals, and an undo, without needing to know in advance what a kernel cache or a spotlight index is. Free-and-technical versus paid-and-guided is the whole comparison.
What each app is actually for
OnyX (by Joël Barrière's Titanium Software) is a maintenance toolbox. It runs macOS maintenance scripts, rebuilds system databases like Spotlight and Launch Services, clears system, user, font, and browser caches, and exposes dozens of hidden system parameters. Each macOS version gets its own matched OnyX build, which is a mark of how carefully it tracks the OS. It is donation-supported and completely free.
Crumb is a disk-space product. It audits the whole Mac read-only, breaks down System Data, finds duplicates and app leftovers, cleans caches and developer junk, and, centrally, explains every item and asks before removing anything, with removals recoverable afterwards. It is built for the person who does not want to become an expert to get their disk back.
The guardrails difference
This is the real dividing line. OnyX gives you power and assumes competence: its options are labeled in system terminology, its actions take effect immediately, and there is no undo. Used knowledgeably, it is excellent; used blindly, it can clear things you wanted kept or leave your Mac rebuilding caches for a day. Crumb inverts those defaults. Nothing is removed without being listed, explained, and approved, and if you regret a removal you can bring it back. Neither approach is wrong; they are for different users. Ask yourself honestly which one you are when something breaks at 11pm.
Price: donations vs one-time
OnyX is free, with donations welcomed, unbeatable economics if its interface does not intimidate you. Crumb has a free version and a one-time $29 purchase with no subscription. What the $29 buys, compared to OnyX, is not raw capability but the guidance: the audit, the explanations, the approvals, the undo, and coverage of space-recovery jobs OnyX does not attempt.
Safety and transparency
Both are legitimate, notarized, and long on integrity; OnyX's twenty-year record speaks for itself, and Crumb's read-only audit and reviewable deletion plan are its founding promise. The practical safety difference is who carries the responsibility. With OnyX, safety depends on the user knowing what each task does. With Crumb, the app carries the explanation burden and keeps a recovery path open. For a shared family Mac, that difference is not academic.
Where OnyX is the better pick
If you are comfortable with system internals, OnyX does things Crumb does not: rebuilding Spotlight and LaunchServices databases, running maintenance scripts, toggling hidden macOS parameters, and fixing odd system misbehavior. It is also the right answer when the budget is zero and the user is technical. Many power users should simply have both installed for different jobs.
Which should you use?
- Use Crumb to recover disk space with explanations and an undo: System Data, caches, duplicates, app leftovers, developer junk.
- Use OnyX for free, expert-level system maintenance: rebuilding databases, maintenance scripts, and deep cache surgery you have researched.
The honest test: read OnyX's maintenance pane. If every option already makes sense to you, it is a gift of a tool, use it. If it reads like another language and your actual problem is a full disk, that is precisely the user Crumb was built for, and its free version will show you what it finds first.