Make heroes count more! One water of life is better than sir littlefinger.
I suggest to Either change their bonuses to be unique like Kvothe or the horadric mage or make their bonus multiplicative, so they mean something the whole game.
You want items? Use littlefinger. You want attack speed? Use Shadow Mane.
The values might need some changes but I think this would be a nice change.
You have a point, but the other difference with these heros and a single potion or item is that the hero effect applies to every tower, not just one.
The other thing to consider is that little finger and shadow mane are beginner level heros. They give you an early boost, but are negligible later. Other heros like horadric mage and jack in the box provide lasting benefits at the expense of a more difficult early game.