Bus and van services can make Ayutthaya a day trip, but the useful comparison is door to door: Bangkok departure point, Ayutthaya drop-off and transport between the ruins.
The Transport Company register lists both Bangkok–Bang Pa-in–Ayutthaya and Bangkok–Ayutthaya routes. That confirms the corridor, not a permanent timetable. Check the official booking channel or terminal notice for your date and avoid assuming every vehicle leaves from the same Bangkok station.
Ayutthaya’s major ruins are spread across and beyond the old city island. A drop-off in town does not create a walkable circuit. Decide whether you will use a bicycle, tuk-tuk or hired car, and agree the route before spending the hottest part of the day negotiating transport.
The train can be easier when its Bangkok station and Ayutthaya arrival fit your day. Compare actual departure points rather than choosing only by journey time.