which one is better
Apples and oranges! Each offers a different experience.
What do you want?
We spent 5 nights on Hamilton island 18months ago. At the time we had a 5 year old and 12 year old. We all absolutely loved it. We stayed in the resort (palm bungalows) which meant a lot of non motorised water equipment was included (standup paddle boards, life jackets, snorking equipment, kayaks etc) you could snorkel off the beach and guided tours were around $15. We saw a lot of sea turtles snorkling straight off the beach and some coral but a day trip was definitely worth while as these reefs had the most amazing coral and sea life. Food and service was great at the resort and we stayed when they offered kids stay and eat free so it wasn't too expensive. It's the kind of place that can be as active or relaxing as you want.
The question should be Port Douglas or Hamilton Island - cairns is not o a beach, and is a regional administrative centre. PD is a lovely little resort town with access to the reef, beach, rainforest and freshwater swimmingholes.
Do you want an "exploring" holiday, or a "laze around the beach drinking cocktsils" holiday?
Agree they are totally different - Cairns is a regional large city with no beach and no reef (you need to spend a day tour to get to outer reef) and Hamilton is a resort island with some high rise and a airport, marina and had a direct hit from Cyclone Debbie (which closed Daydream and Hayman Island until next year).
As above, you really should compare Port Douglas (beach, day trip to reef, marina, day trip to rainforest) and Hamilton Is.
Depends on what you want to do ?
Agree, it depends on what you want. Hamilton Island (HI) is an island it does have an island feel and spectacular views. It only has a handful of high rise buildings and to get around you have to use a golf buggy or bus shuttle. Hamilton Island is usually known as the rich people's Summer playground so bear in mind the cost when eating out at the restaurants as they're all pretty much at the same price point and IMO, not much range. Having said that, we really enjoyed our trip but that's because we had kids at the time. I would not visit HI again unless I was staying at an adults only resort as they take over all the 4 pools.
As others have mentioned it depends on your interests & also what experience u are after.
It would be better if you state your interests before we give you suggestions.
I find questions like this useless as the poster cant even be bothered to put the question into context. So for example- they may love nude sea bathing and as they have not said this is something which interests them - they could find that indeed its inappropriate and fraught with danger in the stinger season / thus neither spot is better. Lol