If you already have enough RAM, adding more does nothing. In this case, increased RAM speed, which still does almost nothing, is preferable.

But going forward, my money is always on RAM needs increasing. The RAM that's enough today probably won't be enough in a few years. That alone is enough reason for me to choose the higher capacity option.

The final option is to sell the excess RAM, and buy a yacht. Or put the money in a savings account and wait for prices to come back down, at least below what you sold it for. Beware that predictions on this are absolutely all over the board, so you'd be taking a risk. But DDR4 is already pretty old, and will disappear soon enough anyway.