I feel like it's neither. For me, a hamburger is a meat or vegetable pattie in a hamburger-shaped bread and a taco is loose mear or vegetables in a soft, flat-pressed bread or in a flat-pressed bread that is fried into a V shape. A hotdog doesn't fit either of those definitions so I consider it a different food.
And depending on where in the world you ask about what hotdogs look like, you could get something completely unlike a hamburger or taco!
For instance in Korea, hotdogs are sausages on sticks that are coated with a pancake-like batter and fried (rather than a sausage on a bun with toppings and rather than a corndog which uses cornmeal in the batter).
Taco because you hold it similar to one, also with a hamburger it is in-between two pieces of bread; whereas with a taco it is one consecutive thing (just like how a hotdog bun is one thing holding in the hotdog itself).