To book a train that originates in Austria, you should use the website of OeBB, the national rail carrier of Austria (www.oebb.at). I was able to enter June 15 and came up with numerous options on that day to Prague, with prices included. If you want to purchase the ticket online, if you want to save money and if you know exactly what train you want to take, then you should select the SparSchiene option. The lowest SparSchiene fare for Vienna to Prague is 29Euro per person. This fare pops up for most of the options for June 15. Of course, as to which is the "best train," direct service (0 changes) is always easiest, isn't it? Do you have a hotel booked in Vienna? It might be interesting to know as you have several options for departure stations for Prague.