It is always nice to have extra flow on hand when needed, with the ability to adjust flow as needed. I have an older xp3, pulled out of storage, on a heavily planted 20l grow out tank and it is working well. After a major trimming I turn back the flow and then steadily increase flow as the plants grow back in throughout the week(s).
The scape will affect the need also. A tank full of carpeting plants will not have the same flow needs as a jungle style. It is nice to have options, depending on need.