For one drink of standard size, a 300ml jug is sufficient, can also use 600ml jug but they better suited to two drinks.
I think the capacity would be measured nominally full
The price/performance variation can come down to things like;
- the handle, is it just a flat bit of steel or shaped for more comfort,
probably a bigger issue if you are using it all day rather typical home use,
- the shape of the spout generally has big impact on performance, a more pronounced spout should be better for any kind of latte artwork, but more expensive to produce,
- the shape of the jug as well, with some shapes being easier to manufacture, but any shape should be able to achieve decent microfoam,
- how much markup the seller puts on their products, no affect on performance, but can affect relative price, etc.