I can only comment Koh Samui v Langkawi
They are very different places. Samui is very touristy, fairly expensive by Thai standards and has been over-developed in the last 5-10 years - but it hasn't gone as far as Phuket. It's still a fun place, great beaches and fairly safe (as long as you don't rent a motorbike). It's a good choice if it's your first trip to Thailand/SE Asia.
Langakwi is the island where nothing ever happens and almost everything shuts down by midnight. Go there for peace, relaxation but don't go for a party. It's perfect for a honeymoon. It's also a tax-free island so things (esp alcohol) are much cheaper than elsewhere in Malaysia.