What is the best site to book a train ticket from Vienna to Prague? Are there different trains/railway options?
Either at OBB (Austrian Railways)- https://www.oebb.at/en/
or Czech Railways (CD), who are usually cheaper.- https://www.cd.cz/
Example the 1110 on 9 August on OBB = 43.10 Spar Scheine, on CD it is 630CZK= 25.53 Euro
You may try RegioJet: https://regiojet.com/
They offer both busses and trains. For trains they have 1st class and three different variants of 2nd class.
Thank you for the information.
What is the difference between those trains in terms of service, convenience, etc?
This link gives detailed information on the choices you have for this route, including photos - https://www.seat61.com/trains-and-routes/prague-to-vienna-by-train.htm