I personally love Thai food because of the strong flavors and spiciness, but I can't imagine eating it too often because it's too rich and sometimes greasy. I choose restaurants through
https://nicelocal.co.uk/london/restaurants/type/thai/ . Vietnamese food, on the hand, feels lighter and cleaner to me (not all dishes, but most) and usually comes with tons of veggies. For example, the Vietnamese canh chua seems a lot lighter than the Thai tom yum. So, generally I pick Vietnamese because at least I can eat it everyday without getting the sick greasy feeling that makes me nauseous, but I love Thai food too!