Some are sweetened, some aren't, the same as soy milk. Of course there is also also rice, coconut, and oatmilks if you are avoiding dairy. They are usually fortified, and may have stabilisers/thickeners/oil etc in them. Remember cows milk has a lot of sugar in too.
Different brands of the same type of milk will taste different, as well as different types of milk obviously. In a smoothie i t probably doesn't matter too much as will be quite masked by the flavours of the smoothie.