The functional difference between liquid and dry is that liquid is just dry ferts watered down so you pay for water. Fine fore small tanks, but a bigger tank it would become pretty expensive.
There is a much larger difference than that. Liquid ferts are great for stem plants because they can pull their nutrients directly from the water column; other plants, like amazon swords, prefer root tabs because they pull their nutrients from the substrate. Also, I don't see the relationship between tank size and fert solution volume when you're making your own. I can prepare my solution so that I'm dosing 5mL into a 5g tank, or I can make it more concentrated so that I'm still only dosing 5mL in a 100g tank.
i believe the dry ferts kfryman is referring to are usually dissolved in water and dosed directly in the water column....
you certainly pay a lot more when buying a fert solution from seachem as opposed to mixing your own ferts.