some goats are louder than others. I find our lamanchas are loud only if they're in heat or they're REALLY hungry. like missed a feeding hungry.
if you get a cow, you need to consider what' you're going to do with all that milk. and you'll need an extra fridge. think about how fast your fridge will fill up on 5-8 gallons a day. if you're not making cheese every other day, you're going to have a lot more than you'll know what to do with.
there are some lower production dairy cows, milking shorthorns, mini-jerseys. sometimes you can find a cull cow from a dairy, one that only has 2 quarters in production instead of 4, and that brings the quantity down. if you get a cow from a dairy, make sure it's one that can be hand milked... not all of them like it and some aren't really good starter cows.
goats vary in production based on size, breed and individual performance. ours produce between a quart and two gallons a day. two gallons is a pretty high volume for a goat, most won't do that much.
if you're looking for weed patrol you might need more than 2 of either choice.