I would say Transmission has simpler UI. Outside of UI, it's also packaged differently, compared to Qbittorrent, Transmission can be installed completely headlessly without dependencies to QT/GTK+X.
In terms of actual features, Transmission is more lightweight. It doesn't support super-seeding, broadcatching, native SOCKS support and doesn't have built-in search engine integration.
In terms of actual features, Transmission is more lightweight. It doesn't support super-seeding, broadcatching, native SOCKS support and doesn't have built-in search engine integration.