With lumber, you can have stronger joints, like mortice and tenon. Plywood joints are really limited to rabbets and you need more pieces to stabilize the corners. It is difficult to find new plywood with a thick outer veneer, so a deep gouge will go right to the inner core layers. A gouge in lumber looks less obvious, unless the wood is stained a strong color. Lumber will give you the option for a nicer looking bed frame and if well built will outlast a plywood bed frame.