I've done quite a bit of research on the topic. I say the train eurarail pass is by far the best choice of the two.
Busabout is not very flexible. It only has pickup and drop off points in major cities. The routes are very few. and I think it costs a tiny bit more than the train. Also if you want to go to say for example south of Rome, Italy, you have to pay extra with busabout. Plus busabout drops you off at certain hostels. That would kinda suck if you couldn't get a reservation at that hostel. And most pick up times for the bus are at like 8am. shitty if you're at a hostel on the other side of town.
With a railpass, you have multiple departure times. Many, many, more routes. way faster. You can sleep alot better in a bed (overnight trains), than in a bus seat. With a train pass you simply have more options and flexability for you and your vacation. lol.
Hope that helped. Enjoy your trip.