Trains from Vienna to Salzburg

Yes, Railjet is fine. It is operated by OEBB, the Austrian National Railway. Their trains go from the Hauptbahnhof. Easy to get to from The Opera on the U1 subway.

Either is fine. Just be aware that they depart from different stations. Both are easy to get to using public transportation.

Be aware that the Westbahn tickets include a seat reservation, wheras with OeBB you must buy it separately.

Westbahn trains depart from Wien Westbahnhof (western train station) [U3, U6] and Wien Hütteldorf [U4],
OeBB trains from Wien Hbf (main train station) [U1] and Wien Meidling [U6].
In brackets there are the subway lines for reaching the respective train station quickly. Which is most convenient depends on your location in Vienna.

Thank you all so much. All your responses were very helpful. Decided to go with Railjet due to proximity to the nearest subway. Thanks again!

Please know that Railjet is the name of the type of train - it is not the name of the company. The company is OBB and the type of train you will take is a Railjet.

I flipped a coin and took Westbahn but they are essentially the same. Since it's private Westbahn will likely not accept passes or tickets for other train lines.

As noted however, you have to make sure you go to the correct train station in Vienna (not a problem in Salzburg).

One thing of note, I bought my tickets from some humans in Vienna and was quizzing them about whether I wanted first or second class. They just shrugged and said start in second and once you're on the train, if you decide you want first, you can upgrade. We did not find it necessary.