Choose Hanoi for the northern route
Hanoi's official tourism page positions Noi Bai airport outside the centre and identifies Ha Long, Sapa and other northern destinations as nearby continuations. The Old Quarter, Hoan Kiem and heritage circuit can support a compact first two days.
Choose Ho Chi Minh City for southern range
Ho Chi Minh City offers a larger, faster urban experience with day-trip links to Cu Chi and the Mekong. Traffic can expand airport and cross-city movement, so a well-chosen hotel area matters more than a long attraction list.
Use open-jaw logic for both
If both cities are essential, arrive in one and depart from the other where the international itinerary permits. Backtracking for the final flight adds cost, one more domestic sector and disruption exposure.
Quick answers
Which city is better for a five-day first trip?
Hanoi if Ha Long is non-negotiable; Ho Chi Minh City if you want one hotel and southern day trips. Da Nang plus Hoi An may be easier than either for a culture-and-coast trip.
Can I visit both cities in five days?
Possible, but usually inefficient. The domestic flight and two airport transfers displace too much usable time unless the trip has a specific business or family reason.