Just got back from a trip to the GBR, can't compare liveaboards as only went on the one while I was there, but I was very impressed with the trip I did. Did the Fly/Dive Cod Hole trip with Mike Ball. 12 dives 3 nights from Lizard Island to Cairns.
The crew was excellent - went out of their way to ensure that you had a great trip. The boat is getting a bit long in the tooth, but very stable and comfortable. Nitrox available, good food, small cabins, but frankly who cares. I am only sleeping in the cabin, I ended up with a "Premium" cabin and would not bother unless you really really want a private bathroom.
The diving was excellent, you can dive whatever profile you want, we had a range of divers from just certified to my buddy who had 50 years diving experience and stopped logging dives years ago. I gather that they are more expensive than others, but if possible check for standby rates, I booked at the last minute and took the last spot on the boat at about 70% of the posted price.
18 divers on the boat with as I recall 8 crew. The dive deck gets a bit hectic and the two best sites were a bit crowded with 18 divers in the water, but after diving on both these sites a number of us went back in the water to snorkle - just could not get enough.