Migros Mobile runs on the Swisscom network, so coverage and 5G are identical to the original. The difference lies in price, service and the size of the roaming packages.
If you only call and use data in Switzerland, Migros Mobile gives you the same network quality for considerably less, plus Cumulus points and advice in a Migros store. Swisscom is worth it if you need the broadest roaming portfolio, bundles with Internet and TV, or the full support of its own shops. Worth noting: on customer satisfaction the two sit close together, see the Peer Score above. The network is the same either way, so at Swisscom you are mainly paying for service scope and the brand.