I have not been to koh Mook, so I can not compare the two islands, or say if there are other restaurants close to Sivalai. But the beach there look really great from the pictures I have seen, and Sivalai get pretty good reviews. A bit more expensive and upscale than Fantasy, I think. If you don`t like big tv showing sport, avoid the bar at Fantasy.. :-)
I posted this on March 26 at the Trang forum, where you also can find many other postings and threads on koh Ngai, Fantasy etc:
We stayed at Koh Hai (Ngai) Fantasy resort recently. A nice place situated at the best part of the beach. The rest of the beach is not very good. Our 2.row sea view bungalow was comfortable, but a bit old. If you want more luxury, go for the new, big beach front bungalows. The restaurant and the bar were ok with very good service. The only minus was the big tv screen in the bar with loud music or football all evening. It was not what we expected at a small and peaceful island. Terrible.... Big, nice swimming pool with a pool bar, but it was too much chlorine. Very strong smell and not good for the skin.
There is no sunset on this side of the island facing the mainland, but don`t miss the sunsrise. It is fantastic!
If you follow the trek to the other side of the island as mentioned above (wear good shoes/sandals and look out for snakes..), turn to the right at Paradise beach. There are two beautifull small beaches with crystal clear water, and a viewpoint at the end of the trek. At Paradise there are some small bungalows and a beach restaurant.