SGN has things pretty centrally located, the two taxi companies of Vinasun and Mai Linh are good and Grab is good too, you can walk pretty easily all around d1 to the major sites (although not that spectacular), and there are some nicer areas/hotels and a backpacker area all in the same vicinity. Plus it is safe, with only real common issue being theft. There are tours and guides readily available. Although again many of the tours are not grand on a global scale of attractions, they are easy and you get what you pay for. Street food is plentiful and food tours are common.

MNL is much bigger and more spread out - the backpacker area has deteriorated, while the nicer areas of Mall of Asia, Makati and the Fort, and Ortigas, have malls but not much else and are a bit of a drive from Intramuros and the old town area. Additionally traffic is way worse, and taxis are less reliable to not have hot meters. I drive a motorbike in MNL, and have ended up via accidental wrong turns in areas I am not comfortable at night. MNL has a lot more crime than SGN, from my outsider/observer point of view. I love Corregidor and the American cemetery more than about anywhere I've been in Asia, and love the Philippines overall, but MNL is just not a tourist city IMO. If OP or anyone is into day trips though, you can make some nice ones from MNL to Pinatubo, Corregidor, Taal, and Anilao/Batangas etc.

I'm not Filipino or Vietnamese, these are just my views having spent time living in both cities. To be fair though, much of my MNL time was quite awhile ago, with recent years just being shorter visits.