15 or 20 gallons: does 5 gallons make a big difference?

[dmiller212]

Honestly, if you're used to managing 6 gallons, you will be pleased with the upgrade either way. You probably also know just what it is that you want, given your prior experience. Totally fine if 5 gallons more isn't important to you.
As mentioned, with a tank this size, 5 gallons is a large percentage, so fairly significant when it comes to maintaining parameters.