I've been both places the differences between Sabah(K.K.) & Phuket are:
*Sabah are not too many HK people but Phuket just like H.K. co'z you can see so many HK ppl in the street or scenic spot.....
*Sabah have a very famous place and called Mount. Kota Kinabalu(K.K.) you can join the local tour if you want to go there.
*Beaches of coz' Phuket beaches are also well-known all over the world but I suggest you better join their local tour for 2-3 island/beaches/day and some islands/beaches are not for HK ppl. only but for foreigner.....don't waste time in Coral Island as there're too many HK ppl Taiwanese......and the beach are not very clean and the coral are not alive.....nothing to see...you can try Phi-Phi island.........
*Food are these 2 places are also delicous Thai food always spicy a bit sweet...use lots of fish sauce.....Malay food have so many choices Satay Hai Nan chicken rice .......Laksa.....those are super yummy..... :em04: seafood in Phuket & Sabah are unlimited supply.
*Scuba diving in Sabah are cheaper than Phuket and if you couldn't dive into the sea you can have a refund but you never meet these case in Phuket.