Well, I did some
Googling around, and came up with the following conclusion:
ご苦労様(ごくろうさま) is used by superiors to thank his/her people lower in rank.
お疲れ様(おつかれさま) is used amongst peers and to people higher in rank.
So a CEO would say ご苦労様 after a meeting, to which the workers could reply with お疲れ様.. Or more formally お疲れ様でした.
I love the word お疲れ様, especially when toasting (instead of Kanpai)..
お疲れ~っす!
Maybe it's just me, but it sounds more "caring" towards each other than a simple "cheers" or "santé".