I've been to both Lizard and Heron Islands. Both are very nice, but different. Lizard caters to a smaller number of guests, is quieter, more upscale, and more expensive. It's the more romantic of the two, with private beaches and dinner for two set up by candlelight on the beach. Service was wonderful, and I would go back in a second.
Heron on the other hand is a much more active island. Many more activities to get involved in, much more wildlife, and more of a "younger" feel to it. Accomodations are simpler, the food was great, and the staff was very helpful and accomodating. It's also less expenseive. I would go back to Heron in a second also - just two very different experiences.
Keep in mind that Heron is much closer to Syndey than Lizard. Lizard would require two more fairly expensive flights to get there - and take a day of travel.
If you're young, like activity, wildlife, and it's your first time to a Barrier Reef Island, I would go with Heron over Lizard. Others have very good things to say about some of the other islands - I haven't been, so can't give you first hand information.
Wherever you end up, I'm sure you will have a great time.