Hey pookie_rabbit, welcome to the group.
Bus to BCN is cheaper by far than the train but less comfortable and a bit slower (about an hour to an hour and a half more).
Enatcar/Alsa busses will get you from Madrid to Barcelona in about 8-8.5 hours, see
MY TRANSPORTATION SECTION for details.
You can check the Alsa bus timetables at
http://www.alsa.es/44/horarios/hor_navega.asp (I always get a kick out of the use of the word "Destiny" instead of "Destination" on the form

Bus is about 22 Euros one way and 40 round trip.
Fastest train right now is the Talgo which takes 7 hours from Madrid to BCN for 42 Euros one way and 67 Euros round trip).
You are more likely to need a reservation on the train ticket but you should be able to get a seat only a few days before in Sept. since it is not that busy then.
Barcelona is an exception to the general rule of the bus being faster than the train is Spain due to tho poor layout of the rail system.
I just bought a ticket one way to Barcelona on the Talgo train and leave Friday morning at 7am to arrive in BCN at 2pm. I have only take a looooooong train in Spain from Granada to Barcelona and it was a sleeper so I can't give my feedback onthe viws along the way.
After my trip this weekend I will post on the scenery on this same thread, I promise. I may even take a "FEW" pictures although it is rare to catch me with my camera in hand
Saludos,
jer...