Perhaps the higher-rated fuel tank has fewer holes for the fuel to leak out of...
If you are thinking of adding extra tanks keep in mind that the fuel cost for a jump scales in proportion to the total mass of the ship hull plus components plus cargo
plus fuel so you will use more fuel to carry the extra fuel along with you. Insert your own Xzibit image here. Since each FSD has a cap on per-jump fuel usage your jump range will also be reduced and you will need to make more jumps to reach your destination.
That doesn't mean that fuel tanks are useless, though. They do save time scooping or docking to refuel, and sometime there are areas of space - even in civilised space - where scooping isn't possible.
Plus with fuel tanks and a fuel transferrer limpet controller you could be a Fuel Rat. Or fuel cat.
