I agree with Wookie, the diving on the GBR is not spectacular and there is way better diving in that area of the world. However, if you are new to diving then diving the GBR while you are there is something I would definitely do. Save the really spectacular places like Komodo, Raj Ampat and PNG for another trip.
With respect to the outer reefs on a liveaboard vs day trips from Port Douglas, no question the liveaboard option is far better. Did both and the difference was significant. The day trips are a long boat ride out to the reef packed in with 30 other divers snorkelers (mostly snorkelers) followed by a dive. Then you wait on the boat for an hour and then do a second dive, and then a long boat ride back to port. The diving is just OK, but if that is your only option and as beginners it will be a great experience. A liveaboard will take you much further out on the reef where there is much more to see and you will be able to do 4 dives a day rather than 2 and skip the long boat ride out every day.