Nelson,
How deep do you need to go? I've made drainage swales with my rollover blade by tilting it severely to one side with the leveling box and dragging it along with the low end of the blade doing the cutting. It takes a few passes in each direction but it makes a nice gentle swale and it leaves very little loose dirt in the bottom. Looks like the tree spade will leave loose dirt in the bottom, and you won't be able to mow across the ditch.
Gabby