I had checked 2 days ago what bus I needed to get back to Vienna. There were lots of seats, but I had some problems with the website, so I decided to get tickets at the bus station, which ended up not happening. I checked again before I went to bed. The bus was only about half-full. I woke up early enough to get to the bus station plenty early- in fact, the ticket window wasn't even open yet. (It opens at 6am). I was super-prepared and second in line when it did open. There were no seats. I made the lady repeat her statement- the bus was completely full and there wasn't a single seat to be had. So much for good planning and plan A. This not only sucked for me, but there was another girl in line behind me who apparently wanted the same thing and also couldn't wait until 3pm to take the next bus.
Fortunately, I had also checked my train options and knew when and where to get a train. We metroed together to the train station, bought train tickets, bought breakfast, and had just enough time to get on the train and get settled before it left. We may have paid a bit more, but I'm guessing we'll get there faster, and I know it's a more comfortable ride. As I type this, she's spread out on 2 seats, sleeping, and I was earlier, as I had all the leg room in the world. The view on the train isn't so much though. Basically, there are a bunch of trees blocking all the views, although occasionally I get to see a farm or village. I guess this is the Hungarian countryside though.
Summary recommendation: take the train unless you're on a tight budget. It was a bit more expensive and only dropped me at the train station, where I had to take another bus to the airport, but it was a much nicer ride than the bus and I was able to get more done since the wifi actually worked. Also, I slept better.