I voted Halong. I traveled 2 times to Vietnam, it is an amazing and wonderful country with friendly local people. Although Halong is touristy, it is worth to travel. You don't forget to spend time to overnight on cruise. Halong Bay is really stunning in night
Thanks to all response, I posted the question on other forums and someone told me that Halong is better than Hoian because it is near Hanoi and suitable with my short trip. Further, they also recommend to Halong-bay-Tours with many cruises for my choice. So, I decide to visit Halong but Halong-Bay-Tours is good for traveling?