The separation in daily work sorts into three honest differences, then the who question answers itself per desk:

Depth of features: the desktop applications carry everything, Online carries the everyday core. Word Online writes, formats and comments capably while mail merge, advanced fields and heavyweight document machinery live desktop only. Excel Online handles ordinary sheets and shared editing while pivot table authoring depth, Power Query and macros belong to the desktop, VBA not existing in the browser at all. PowerPoint Online assembles and presents decks while the finer animation and design tooling sits installed. The pattern holds across the suite: consumption and everyday creation online, power work installed.

Files and offline: desktop applications open anything from anywhere, network shares, local folders, attachments double clicked and keep working when the internet does not. Online works on files living in OneDrive or SharePoint through a live connection, full stop, which for an office with a file server tradition is frequently the deciding difference before features even enter it.

Where Online wins outright: simultaneous co editing was born there and remains smoothest there, nothing installs or updates on the machines and any borrowed or home device becomes a workstation through a browser tab. The plan arithmetic then writes itself desk by desk: staff living in heavy Excel, mail merge or macro territory need desktop plans, staff whose reality is mail, light documents and shared sheets thrive on Online plans at the lower price and the mixed answer, most desks desktop with a few Online, is the normal shape for twenty people rather than a compromise. Plans upgrade per user later, so doubt resolves toward starting lighter and promoting the desks that hit the walls.