OBB or Westbahn Train?

We just took that same route at the beginning of June. We used IBB because their app seemed to be more user friendly, and the train times worked better for us. great trains, great service, loved it!

The quality of trains and the travel time are about the same. Westbahn tickets include a seat reservation, with OeBB you have to book it separately.

Westbahn and OeBB use different train stations in Vienna. So choose the train station which is most convenient for you. (Nevertheless, because of quality of public transport all train stations are easily reachable.)

Westbahn departs/arrives at Wien West (western train station) and Wien Hütteldorf.
OeBB departs/arrives at Wien Hbf (main train station) and Wien Meidling.

It is always worth to compare the fares offered by both companies for a certain date.

As noted find the station that works best for you and see who goes there.

Also check with the Man in Seat 61, I'm pretty sure he will say it's a wash. Just beware that passes don't work on Westbahn, they made several announcements before the doors closed.

Just beware that passes don't work on Westbahn ...

This is not correct (anymore).

Holder of Eurail/Interrail passes will be seated in the Standard Class (even if their pass is for First Class); a paid upgrade is possible.

Austria has several Unified Fare Compounds (Verkehrsverbund), i.e., large areas around big cities having one ticketing system for all means of public transport, no matter which company is the operator. Westbahn is now member of all of them, e.g., VOR (Vienna), OÖVV (Linz), SVV (Salzburg), TVV (Innsbruck), etc. Consequently, all ticket issued by them are valid on Westbahn.

The same is true for the Klimaticket, the one-year pass valid on all public transport throughout the whole country of Austria.