Hello! Thank you for visiting the forums. You can sign up or log in using the button on the top right of your screen. If you do not have an account consider signing up and joining the forums community!
Without a crop hopper, string loses WAY too much cactus that it becomes the most inefficient you could possibly make. If you're not worried about costs then check out /visit thecheken for the most efficient cactus farm you can make. Out of the two you've listed however, on this server... Fence wins out 100%