A toxic gas that leaves a residue you have to air out, vs. a beam of radiation that leaves nothing behind except a changed material.
Both are dominant terminal sterilization methods for medical devices, and both reliably achieve the sterility assurance level regulators require. Ethylene oxide (EtO) is a low-temperature gas process: EtO molecules diffuse into and around packaged devices and alkylate microbial DNA, but the gas itself is toxic, flammable, and carcinogenic, so every sterilized load must go through an aeration cycle to purge residual EtO and its reaction byproducts to safe limits before the device can ship. Gamma irradiation uses high-energy photons from a Cobalt-60 source to directly damage microbial DNA — it leaves no chemical residual at all, but the same ionizing energy that kills microorganisms can break polymer chains, and certain materials — most notably ultra-high-molecular-weight polyethylene (UHMWPE) used in joint-replacement bearing surfaces — become measurably more brittle and oxidation-prone after a gamma dose.