A crown is a more involved procedure than an onlay. A crown is a porcelain cap that the dentist fits over a tooth.
First, they remove the old filling and any decay. Then, they have to remove even more of the tooth (the healthy part) so the crown fits on top.
With porcelain onlays, you have a solution that puts off needing a crown.
If you have a dental onlay installed to replace a broken cusp, you’ll get many years of use out of it. When it has finally been ground down, then a crown is a good way to restore the tooth the second time.