they are the same....
disc at once means you cannot write to it again
the other option, track at once, always you to leave the session open, so you can write more later
for audio i would go with disc at once (the seconds between the songs is much less), and for data i would go with track at once (with session left open), unless of course you are distributing your stuff, and dont' want people to use the cd (in which case you use disc at once)
hope this helps