Johor Bahru is the state capital of the state of Johor and the second largest metropolitan area in Malaysia. Most tourists don't stop here and just pass through the city on their way to or out of Singapore. JB is a major local transportation hub however. Since it is just across the causeway from Singapore, some travellers use it as a hub to visit Singapore from here (lower accommodation prices) or to board the train, bus or airplane.
Langkawi is an archipelago of 99 islands in the Andaman Sea, some 30 km off the mainland coast of northwestern Malaysia. The islands are a part of Malaysia’s Kedah state, but are adjacent to the Thai border. By far the largest of the islands is the eponymous Pulau Langkawi with a population of about 45,000. The island is a declared duty-free zone.
The peak season is from December to March when moonson at the eastcoast shuts down most of the Islands there (Perhentian etc.). This makes Langkawi a good island alternative during that time period. Generally though, Langkawi can be visited all year around.