Also, the point above about re-used. for calculations we say "calculate once, reference many times". The same applies for structures. If you need to repeat the line items many times, a list will be better
But the other aspect to bear in mind is what is the logic for Volume1, Volume2, Volume3?.
If the logic is the same or similar then a list would be simpler; One tip here. Don't write a formula like this:
IF ITEM(VolumeTypes)=VolumeTypes.Volume1 THEN xx*yyy ELSE IF ITEM(VolumeTypes)=Volume2 THEN xx/yyy ELSE.........
If is better to have a module dimensioned by Volume and have a boolean flag signifying the logic type; it is much more flexible and efficient (especially if some volume types hare the same logic)
If each type has a very different type of logic then it might be better to have line items; remember you can always bring numeric line items together using a line item subset