Musken

It is of course personal, but I find that spending more time in another place instead of doing two large cities will give you more of an interesting holiday. The two cities are different, but not as different as a big city is from a smaller town/beach area. I found Hanoi to be interesting as such and quite charming with a number of things to see plus great food/street food, while HCMC was less interesting in itself (but you can use that city as a base to go to the Cu Chi tunnels and the Mekong delta). Of course up to you, and if you have enough time for everything, please feel free.