The right answer depends on how many cities you want and how much of the day you want back.
Rail: fast, flexible, city-centre to city-centre
High-speed routes between major European cities are often faster than flying once you count airport time. Rome to Florence is ninety minutes. Paris to London is two hours twenty. You arrive in the middle of the city, not forty kilometres outside it.
Rail: the trade-offs
Reserved seats sell out on scenic and high-speed routes. Luggage is yours to manage on and off. Costs rise sharply if you book late, so a rail-based itinerary needs to be fixed earlier than a coach one.
Coach: everything handled
Luggage loaded for you, hotels on the route, a guide with you throughout, and countryside between cities that rail tunnels straight past. For a first trip to Europe covering five or more cities, a coach tour removes almost all the decisions.
Coach: the trade-offs
Fixed departure times, fixed group pace, and some long driving days. Hotels are often on the edge of the city rather than the centre.
How we usually split it
Two or three cities: rail, with private transfers at each end. Five or more cities across multiple countries: coach. Switzerland: rail, always — the trains are the attraction.