Potatoes do have a better return on investment per plant but because of the difference in growth speed, parsnips actually give you a better return if you replant all profits. This is because the growth cycle makes your plot grow much much faster with parsnips.

That assumes that your plot keeps growing all season ofc.

Edit: I have a full valculation of this, but I’m not near my pc. If you want I can look for it later today