Hey there,
It’s always this question: Hanoi or Saigon. I’ll tell you a secret before answering. The most correct answer is visiting both and finding yourself the answer ;). However, I’ll give you some information so that you can see which one will be suitable for your type.

• About geography and climate:

Hanoi locates in the North of Vietnam, surround by Red river and have many lakes. The city’s climate is classified as “warm, humid subtropical” – which essentially means cool, dry winters and hot, wet summers. 
Saigon’s located in the South, lies in the coast of Mekong river (Saigon river) with tropical climate which means this city has 2 seasons: dry season and wet season with hot temperature all year around.

• About culture:

Hanoi is the capital containing 1000 years of literature when Saigon is the most energetic city in Vietnam which dated back to 300 years ago.
Hanoi has slower life than Saigon. I can tell that Hanoi was affected by French culture when Saigon was affected by American.

• About sightseeing:

I think that Hanoi has more sightseeing than Saigon.
Since most of the fighting during the Vietnam War occurred in the south, Saigon is the place to see war-era museums and relics, such as the Ho Chi Minh City Museum, the Revolutionary Museum, the Museum of Southeastern Armed Forces, the War Remnants Museum and the hauntingly evocative Cu Chi Tunnels, used by guerilla fighters.
However, Hanoi contains 100 years history which has left Hanoi with an excellent collection of museums and monuments to keep the visitor busy. Besides those mentioned above, you can also visit the mausoleum and stilt house of Ho Chi Minh, North Vietnam’s charismatic wartime leader. The Old Quarter, meanwhile, preserves its original street layout from pre-colonial times and is a wonderful place to eat and shop – with small artisans and merchants, cafes, restaurants and bars.
Hanoi

Saigon
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• About Food and accommodation:

Both has too various food that I can’t compare in detail but in Saigon, food’s sweeter than in Hanoi. Hanoi’s famous for Pho (rice noodle) which everybody insist in trying. Saigon, well, I’ll say that I enjoy fresh fruit most.
The price for food and accommodation isn’t different much