If you're looking to do floating plants, then I would imagine that the surface area of the water would determine how many floating plants you can have, which would determine how much of a nitrate soak you can have. For example, a 20 gallon long would have a higher capacity to hold more floating plants than say a 20 gallon hex. it largely depends on your stocking levels, your water changes, your plants, high tech/low tech etc.
I guess what I'm trying to say is your question is too open ended with too many variables. You won't be able to get a good answer.
Love your tanks and they will love you back