Hi there,
I have worked on both Phi Phi and Phuket for around 10 years, and the easiest way to get to Hin Daeng is via Phi Phi on a speed boat. The trip takes around an hour anda half and costs around 4000 Baht. There are literally dozens and dozens of dive shops on Phi Phi who run the trip, usually on the same boats, so don't worry about booking before you go, there will always be spaces available. I would recommend Blue View Divers if you do decide to go there.
From Phuket no company runs a regular trip to Hin Daeng, but another option would be to do these sites via a liveaboard. There are boats that do a southern Andaman safari and dive Hin Daeng/Hin Muang, Koh Haa, Koh Phi Phi and a few other spots. This would be the best, most comfortable option to dive these sites.
I dived there in May and had some excellent encounters with large Manta Rays.
Have a great trip with whatever option you decide to go with.