If comfort is a priority take the train. Even though it's slower on this routen and usually doesn't have wifi or other services, it's still more comfortable than buses.
Check timetables on the railway company site https://www.cfrcalatori.ro/en ; you can also book tickets there.
There's not that much difference between first and second class.
The availability of buses with toilets, wifi or other amenities varies. That's not their main attraction in any case, it's just that on this specific route they're more frequent than trains and faster.
You can check schedules on http://www.autogari.ro/Transport/Bucuresti-Sibiu?lang=en
Edited: 6 years ago