Vienna to Budapest by Train or FixBus?

I would just take the train. Why add complications and increase your carbon footprint in addition ?

If you are considering the Flix bus because of cost, have you compared buying your train ticket on the mav site instead of obb?

After reading all the negatives on TripAdvisor I think I will pass (https://www.tripadvisor.com/Attraction_Review-g190454-d17699744-Reviews-or10-Flixbus-Vienna.html) I am surprised RS book even recommended it.

Here are my comments on it comparing train with bus, clipped from a trip report I did on this forum about an April trip to Budapest, Vienna and Bratislava:
"Train from Budapest to Vienna. Much helpful info on train travel on "man in seat 61" website. Bought tix from website called thetrainline.com that was mentioned on seat 61. Did this cost a bit more than the official site? Maybe, but if so not by much, and it was very convenient. Got an email from OBB (Austrian rail) in German a few days before departure, which had me worried, but turned out it was saying, with Austrian efficiency, that our arrival time had changed from 12:21 pm to 12:18 pm! Ok, can live with that! Train (on a Saturday morning was full and all (or mostly all) seats reserved -- glad we spent the extra couple euros for seat reservations. Return to Budapest by Flixbus (kind of like their Megabus). Why not train? Because I had wanted to go direct to Budapest airport, but that bus didn't leave at a good time, so just booked it instead to Budapest main bus station (Nepliget). When we saw the grim little waiting room at Vienna Erdburg bus station, I thought, what have I got us into, but the bus was on time, comfortable, about same length of time as train, and half the price of train."
I should add that although the Budapest bus station is a bit farther from the center of the city than Keleti train station, that was fine with us as we were going to the airport (to stay at a hotel there before catching an early flight) and the bus station was actually closer to that than Keleti. We found a few taxis waiting outside Nepliget and took one of those, so I don't know about public transit options.

Here are generally positive impressions of Flixbus from a current post on the Transportation
forum regarding Italy:

https://community.ricksteves.com/travel-forum/transportation/anyone-take-a-bus-in-italy

Personally, I don't use Flixbus, because this company exploits their drivers, paying the minimum wage, extending the maximum driving and worktime by applying all kind of tricks. Otherwise these low fares would not be possible.

Personally I will never take a bus on a route where a good train service exists. For one thing I do not like the fact that these companies cherry pick the good routes, and by competing with the railways, which offer a comprehensive network, make the whole system worse for everyone. It is a good thing that for example Switzerland does not permit competition on domestic routes. (They cannot do anything on international routes unfortunately. I would just ban Flixbus et al if I was in charge).

Secondly a bus is never going to be as comfortable ad a train. Bus seats are narrower. Leg room is less, and getting up and stretching your legs is not really an option. And the trains have a restaurant car!.

As a frequent Flixbus user between Vienna and Zagreb (the train line is slower and takes more time), I can say that when there is a faster or equally fast train route, there is no question that one would take a train over a bus. The bus seats are very tight, there is very little leg room, there can be smells and cleanliness issues and bus station toilets are the absolute worst (which I only use if it is an emergency). On a train, there is a lot of space even in 2nd class, you can walk around, you can eat in a dining car and use much nicer toilets on demand

Unless you're on an extremely tight budget I would not recommend Flixbus. The train is faster and, as many have mentioned, a lot more comfortable. Expecially the Railjets are very nice. And if you buy the tickets in advance the train is not expensive.