I would say that depends on your needs and your plans. How much
milk does your family go through? Are you going to be using it or selling it? CAN you
sell it or sell "shares" as they say?
I think if you just want a family use amount of milk than depending on family size a goat is probably sufficient. They are also easier to handle as far as size goes and are cheaper. If you want to sell a cow may be more worth your while.
Facilities also need to be considered. I know for my acreage I plan on getting goats but my fencing isn't goat safe, it's cow safe. So I'm going to have to
fence before getting a goat. I also plan on having 2 goats and rotating breedings so the same goat isn't giving birth each year. Anyway, just what is going to work for me since I'm not planning on selling milk and only need 2 gallons for drinking and some excess for cheese and etc. a week.
Either way, good luck!